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Network Working Group                                   A. Phillips, Ed.
Request for Comments: 5646                                        Lab126
BCP: 47                                                    M. Davis, Ed.
Obsoletes: 4646                                                   Google
Category: Best Current Practice                           September 2009


                     Tags for Identifying Languages

Abstract

   This document describes the structure, content, construction, and
   semantics of language tags for use in cases where it is desirable to
   indicate the language used in an information object.  It also
   describes how to register values for use in language tags and the
   creation of user-defined extensions for private interchange.

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Language Tag . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.1.  Formatting of Language Tags  . . . . . . . . . . . . .  6
     2.2.  Language Subtag Sources and Interpretation . . . . . . . .  8
       2.2.1.  Primary Language Subtag . . . . . . . . . . . . . . . . 9
       2.2.2.  Extended Language Subtags  . . . . . . . . . . . . . . 11
       2.2.3.  Script Subtag  . . . . . . . . . . . . . . . . . . . . 12
       2.2.4.  Region Subtag  . . . . . . . . . . . . . . . . . . . . 13
       2.2.5.  Variant Subtags  . . . . . . . . . . . . . . . . . . . 15
       2.2.6.  Extension Subtags  . . . . . . . . . . . . . . . . . . 16
       2.2.7.  Private Use Subtags  . . . . . . . . . . . . . . . . . 18
       2.2.8.  Grandfathered and Redundant Registrations  . . . . . . 18
       2.2.9.  Classes of Conformance . . . . . . . . . . . . . . . . 19
   3.  Registry Format and Maintenance  . . . . . . . . . . . . . . . 21
     3.1.  Format of the IANA Language Subtag Registry  . . . . . . . 21
       3.1.1.  File Format  . . . . . . . . . . . . . . . . . . . . . 21
       3.1.2.  Record and Field Definitions . . . . . . . . . . . . . 23
       3.1.3.  Type Field . . . . . . . . . . . . . . . . . . . . . . 26
       3.1.4.  Subtag and Tag Fields  . . . . . . . . . . . . . . . . 26
       3.1.5.  Description Field  . . . . . . . . . . . . . . . . . . 26
       3.1.6.  Deprecated Field . . . . . . . . . . . . . . . . . . . 28
       3.1.7.  Preferred-Value Field  . . . . . . . . . . . . . . . . 28
       3.1.8.  Prefix Field . . . . . . . . . . . . . . . . . . . . . 31
       3.1.9.  Suppress-Script Field  . . . . . . . . . . . . . . . . 32
       3.1.10. Macrolanguage Field  . . . . . . . . . . . . . . . . . 32
       3.1.11. Scope Field  . . . . . . . . . . . . . . . . . . . . . 33
       3.1.12. Comments Field . . . . . . . . . . . . . . . . . . . . 34
     3.2.  Language Subtag Reviewer . . . . . . . . . . . . . . . . . 35
     3.3.  Maintenance of the Registry  . . . . . . . . . . . . . . . 35
     3.4.  Stability of IANA Registry Entries . . . . . . . . . . . . 36
     3.5.  Registration Procedure for Subtags . . . . . . . . . . . . 41
     3.6.  Possibilities for Registration . . . . . . . . . . . . . . 46
     3.7.  Extensions and the Extensions Registry . . . . . . . . . . 49
     3.8.  Update of the Language Subtag Registry . . . . . . . . . . 52
     3.9.  Applicability of the Subtag Registry . . . . . . . . . . . 52
   4.  Formation and Processing of Language Tags  . . . . . . . . . . 53
     4.1.  Choice of Language Tag . . . . . . . . . . . . . . . . . . 53
       4.1.1.  Tagging Encompassed Languages  . . . . . . . . . . . . 58
       4.1.2.  Using Extended Language Subtags  . . . . . . . . . . . 59
     4.2.  Meaning of the Language Tag  . . . . . . . . . . . . . . . 61
     4.3.  Lists of Languages . . . . . . . . . . . . . . . . . . . . 63
     4.4.  Length Considerations  . . . . . . . . . . . . . . . . . . 63
       4.4.1.  Working with Limited Buffer Sizes  . . . . . . . . . . 64
       4.4.2.  Truncation of Language Tags  . . . . . . . . . . . . . 65
     4.5.  Canonicalization of Language Tags  . . . . . . . . . . . . 66



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     4.6.  Considerations for Private Use Subtags . . . . . . . . . . 68
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 69
     5.1.  Language Subtag Registry . . . . . . . . . . . . . . . . . 69
     5.2.  Extensions Registry  . . . . . . . . . . . . . . . . . . . 71
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 71
   7.  Character Set Considerations . . . . . . . . . . . . . . . . . 72
   8.  Changes from RFC 4646  . . . . . . . . . . . . . . . . . . . . 73
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 76
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 76
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 78
   Appendix A.  Examples of Language Tags (Informative) . . . . . . . 80
   Appendix B.  Examples of Registration Forms  . . . . . . . . . . . 82
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 83

1.  Introduction

   Human beings on our planet have, past and present, used a number of
   languages.  There are many reasons why one would want to identify the
   language used when presenting or requesting information.

   The language of an information item or a user's language preferences
   often need to be identified so that appropriate processing can be
   applied.  For example, the user's language preferences in a Web
   browser can be used to select Web pages appropriately.  Language
   information can also be used to select among tools (such as
   dictionaries) to assist in the processing or understanding of content
   in different languages.  Knowledge about the particular language used
   by some piece of information content might be useful or even required
   by some types of processing, for example, spell-checking, computer-
   synthesized speech, Braille transcription, or high-quality print
   renderings.

   One means of indicating the language used is by labeling the
   information content with an identifier or "tag".  These tags can also
   be used to specify the user's preferences when selecting information
   content or to label additional attributes of content and associated
   resources.

   Sometimes language tags are used to indicate additional language
   attributes of content.  For example, indicating specific information
   about the dialect, writing system, or orthography used in a document
   or resource may enable the user to obtain information in a form that
   they can understand, or it can be important in processing or
   rendering the given content into an appropriate form or style.

   This document specifies a particular identifier mechanism (the
   language tag) and a registration function for values to be used to




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   form tags.  It also defines a mechanism for private use values and
   future extensions.

   This document replaces [RFC4646] (which obsoleted [RFC3066] which, in
   turn, replaced [RFC1766]).  This document, in combination with
   [RFC4647], comprises BCP 47.  For a list of changes in this document,
   see Section 8.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  The Language Tag

   Language tags are used to help identify languages, whether spoken,
   written, signed, or otherwise signaled, for the purpose of
   communication.  This includes constructed and artificial languages
   but excludes languages not intended primarily for human
   communication, such as programming languages.

2.1.  Syntax

   A language tag is composed from a sequence of one or more "subtags",
   each of which refines or narrows the range of language identified by
   the overall tag.  Subtags, in turn, are a sequence of alphanumeric
   characters (letters and digits), distinguished and separated from
   other subtags in a tag by a hyphen ("-", [Unicode] U+002D).

   There are different types of subtag, each of which is distinguished
   by length, position in the tag, and content: each subtag's type can
   be recognized solely by these features.  This makes it possible to
   extract and assign some semantic information to the subtags, even if
   the specific subtag values are not recognized.  Thus, a language tag
   processor need not have a list of valid tags or subtags (that is, a
   copy of some version of the IANA Language Subtag Registry) in order
   to perform common searching and matching operations.  The only
   exceptions to this ability to infer meaning from subtag structure are
   the grandfathered tags listed in the productions 'regular' and
   'irregular' below.  These tags were registered under [RFC3066] and
   are a fixed list that can never change.

   The syntax of the language tag in ABNF [RFC5234] is:

 Language-Tag  = langtag             ; normal language tags
               / privateuse          ; private use tag
               / grandfathered       ; grandfathered tags





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 langtag       = language
                 ["-" script]
                 ["-" region]
                 *("-" variant)
                 *("-" extension)
                 ["-" privateuse]

 language      = 2*3ALPHA            ; shortest ISO 639 code
                 ["-" extlang]       ; sometimes followed by
                                     ; extended language subtags
               / 4ALPHA              ; or reserved for future use
               / 5*8ALPHA            ; or registered language subtag

 extlang       = 3ALPHA              ; selected ISO 639 codes
                 *2("-" 3ALPHA)      ; permanently reserved

 script        = 4ALPHA              ; ISO 15924 code

 region        = 2ALPHA              ; ISO 3166-1 code
               / 3DIGIT              ; UN M.49 code

 variant       = 5*8alphanum         ; registered variants
               / (DIGIT 3alphanum)

 extension     = singleton 1*("-" (2*8alphanum))

                                     ; Single alphanumerics
                                     ; "x" reserved for private use
 singleton     = DIGIT               ; 0 - 9
               / %x41-57             ; A - W
               / %x59-5A             ; Y - Z
               / %x61-77             ; a - w
               / %x79-7A             ; y - z

 privateuse    = "x" 1*("-" (1*8alphanum))

 grandfathered = irregular           ; non-redundant tags registered
               / regular             ; during the RFC 3066 era

 irregular     = "en-GB-oed"         ; irregular tags do not match
               / "i-ami"             ; the 'langtag' production and
               / "i-bnn"             ; would not otherwise be
               / "i-default"         ; considered 'well-formed'
               / "i-enochian"        ; These tags are all valid,
               / "i-hak"             ; but most are deprecated
               / "i-klingon"         ; in favor of more modern
               / "i-lux"             ; subtags or subtag
               / "i-mingo"           ; combination



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               / "i-navajo"
               / "i-pwn"
               / "i-tao"
               / "i-tay"
               / "i-tsu"
               / "sgn-BE-FR"
               / "sgn-BE-NL"
               / "sgn-CH-DE"

 regular       = "art-lojban"        ; these tags match the 'langtag'
               / "cel-gaulish"       ; production, but their subtags
               / "no-bok"            ; are not extended language
               / "no-nyn"            ; or variant subtags: their meaning
               / "zh-guoyu"          ; is defined by their registration
               / "zh-hakka"          ; and all of these are deprecated
               / "zh-min"            ; in favor of a more modern
               / "zh-min-nan"        ; subtag or sequence of subtags
               / "zh-xiang"

 alphanum      = (ALPHA / DIGIT)     ; letters and numbers

                        Figure 1: Language Tag ABNF

   For examples of language tags, see Appendix A.

   All subtags have a maximum length of eight characters.  Whitespace is
   not permitted in a language tag.  There is a subtlety in the ABNF
   production 'variant': a variant starting with a digit has a minimum
   length of four characters, while those starting with a letter have a
   minimum length of five characters.

   Although [RFC5234] refers to octets, the language tags described in
   this document are sequences of characters from the US-ASCII [ISO646]
   repertoire.  Language tags MAY be used in documents and applications
   that use other encodings, so long as these encompass the relevant
   part of the US-ASCII repertoire.  An example of this would be an XML
   document that uses the UTF-16LE [RFC2781] encoding of [Unicode].

2.1.1.  Formatting of Language Tags

   At all times, language tags and their subtags, including private use
   and extensions, are to be treated as case insensitive: there exist
   conventions for the capitalization of some of the subtags, but these
   MUST NOT be taken to carry meaning.

   Thus, the tag "mn-Cyrl-MN" is not distinct from "MN-cYRL-mn" or "mN-
   cYrL-Mn" (or any other combination), and each of these variations




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   conveys the same meaning: Mongolian written in the Cyrillic script as
   used in Mongolia.

   The ABNF syntax also does not distinguish between upper- and
   lowercase: the uppercase US-ASCII letters in the range 'A' through
   'Z' are always considered equivalent and mapped directly to their US-
   ASCII lowercase equivalents in the range 'a' through 'z'.  So the tag
   "I-AMI" is considered equivalent to that value "i-ami" in the
   'irregular' production.

   Although case distinctions do not carry meaning in language tags,
   consistent formatting and presentation of language tags will aid
   users.  The format of subtags in the registry is RECOMMENDED as the
   form to use in language tags.  This format generally corresponds to
   the common conventions for the various ISO standards from which the
   subtags are derived.

   These conventions include:

   o  [ISO639-1] recommends that language codes be written in lowercase
      ('mn' Mongolian).

   o  [ISO15924] recommends that script codes use lowercase with the
      initial letter capitalized ('Cyrl' Cyrillic).

   o  [ISO3166-1] recommends that country codes be capitalized ('MN'
      Mongolia).

   An implementation can reproduce this format without accessing the
   registry as follows.  All subtags, including extension and private
   use subtags, use lowercase letters with two exceptions: two-letter
   and four-letter subtags that neither appear at the start of the tag
   nor occur after singletons.  Such two-letter subtags are all
   uppercase (as in the tags "en-CA-x-ca" or "sgn-BE-FR") and four-
   letter subtags are titlecase (as in the tag "az-Latn-x-latn").

   Note: Case folding of ASCII letters in certain locales, unless
   carefully handled, sometimes produces non-ASCII character values.
   The Unicode Character Database file "SpecialCasing.txt"
   [SpecialCasing] defines the specific cases that are known to cause
   problems with this.  In particular, the letter 'i' (U+0069) in
   Turkish and Azerbaijani is uppercased to U+0130 (LATIN CAPITAL LETTER
   I WITH DOT ABOVE).  Implementers SHOULD specify a locale-neutral
   casing operation to ensure that case folding of subtags does not
   produce this value, which is illegal in language tags.  For example,
   if one were to uppercase the region subtag 'in' using Turkish locale
   rules, the sequence U+0130 U+004E would result, instead of the
   expected 'IN'.



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2.2.  Language Subtag Sources and Interpretation

   The namespace of language tags and their subtags is administered by
   the Internet Assigned Numbers Authority (IANA) according to the rules
   in Section 5 of this document.  The Language Subtag Registry
   maintained by IANA is the source for valid subtags: other standards
   referenced in this section provide the source material for that
   registry.

   Terminology used in this document:

   o  "Tag" refers to a complete language tag, such as "sr-Latn-RS" or
      "az-Arab-IR".  Examples of tags in this document are enclosed in
      double-quotes ("en-US").

   o  "Subtag" refers to a specific section of a tag, delimited by a
      hyphen, such as the subtags 'zh', 'Hant', and 'CN' in the tag "zh-
      Hant-CN".  Examples of subtags in this document are enclosed in
      single quotes ('Hant').

   o  "Code" refers to values defined in external standards (and that
      are used as subtags in this document).  For example, 'Hant' is an
      [ISO15924] script code that was used to define the 'Hant' script
      subtag for use in a language tag.  Examples of codes in this
      document are enclosed in single quotes ('en', 'Hant').

   Language tags are designed so that each subtag type has unique length
   and content restrictions.  These make identification of the subtag's
   type possible, even if the content of the subtag itself is
   unrecognized.  This allows tags to be parsed and processed without
   reference to the latest version of the underlying standards or the
   IANA registry and makes the associated exception handling when
   parsing tags simpler.

   Some of the subtags in the IANA registry do not come from an
   underlying standard.  These can only appear in specific positions in
   a tag: they can only occur as primary language subtags or as variant
   subtags.

   Sequences of private use and extension subtags MUST occur at the end
   of the sequence of subtags and MUST NOT be interspersed with subtags
   defined elsewhere in this document.  These sequences are introduced
   by single-character subtags, which are reserved as follows:

   o  The single-letter subtag 'x' introduces a sequence of private use
      subtags.  The interpretation of any private use subtag is defined





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      solely by private agreement and is not defined by the rules in
      this section or in any standard or registry defined in this
      document.

   o  The single-letter subtag 'i' is used by some grandfathered tags,
      such as "i-default", where it always appears in the first position
      and cannot be confused with an extension.

   o  All other single-letter and single-digit subtags are reserved to
      introduce standardized extension subtag sequences as described in
      Section 3.7.

2.2.1.  Primary Language Subtag

   The primary language subtag is the first subtag in a language tag and
   cannot be omitted, with two exceptions:

   o  The single-character subtag 'x' as the primary subtag indicates
      that the language tag consists solely of subtags whose meaning is
      defined by private agreement.  For example, in the tag "x-fr-CH",
      the subtags 'fr' and 'CH' do not represent the French language or
      the country of Switzerland (or any other value in the IANA
      registry) unless there is a private agreement in place to do so.
      See Section 4.6.

   o  The single-character subtag 'i' is used by some grandfathered tags
      (see Section 2.2.8) such as "i-klingon" and "i-bnn".  (Other
      grandfathered tags have a primary language subtag in their first
      position.)

   The following rules apply to the primary language subtag:

   1.  Two-character primary language subtags were defined in the IANA
       registry according to the assignments found in the standard "ISO
       639-1:2002, Codes for the representation of names of languages --
       Part 1: Alpha-2 code" [ISO639-1], or using assignments
       subsequently made by the ISO 639-1 registration authority (RA) or
       governing standardization bodies.

   2.  Three-character primary language subtags in the IANA registry
       were defined according to the assignments found in one of these
       additional ISO 639 parts or assignments subsequently made by the
       relevant ISO 639 registration authorities or governing
       standardization bodies:

       A.  "ISO 639-2:1998 - Codes for the representation of names of
           languages -- Part 2: Alpha-3 code - edition 1" [ISO639-2]




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       B.  "ISO 639-3:2007 - Codes for the representation of names of
           languages -- Part 3: Alpha-3 code for comprehensive coverage
           of languages" [ISO639-3]

       C.  "ISO 639-5:2008 - Codes for the representation of names of
           languages -- Part 5: Alpha-3 code for language families and
           groups" [ISO639-5]

   3.  The subtags in the range 'qaa' through 'qtz' are reserved for
       private use in language tags.  These subtags correspond to codes
       reserved by ISO 639-2 for private use.  These codes MAY be used
       for non-registered primary language subtags (instead of using
       private use subtags following 'x-').  Please refer to Section 4.6
       for more information on private use subtags.

   4.  Four-character language subtags are reserved for possible future
       standardization.

   5.  Any language subtags of five to eight characters in length in the
       IANA registry were defined via the registration process in
       Section 3.5 and MAY be used to form the primary language subtag.
       An example of what such a registration might include is the
       grandfathered IANA registration "i-enochian".  The subtag
       'enochian' could be registered in the IANA registry as a primary
       language subtag (assuming that ISO 639 does not register this
       language first), making tags such as "enochian-AQ" and "enochian-
       Latn" valid.

       At the time this document was created, there were no examples of
       this kind of subtag.  Future registrations of this type are
       discouraged: an attempt to register any new proposed primary
       language MUST be made to the ISO 639 registration authority.
       Proposals rejected by the ISO 639 registration authority are
       unlikely to meet the criteria for primary language subtags and
       are thus unlikely to be registered.

   6.  Other values MUST NOT be assigned to the primary subtag except by
       revision or update of this document.

   When languages have both an ISO 639-1 two-character code and a three-
   character code (assigned by ISO 639-2, ISO 639-3, or ISO 639-5), only
   the ISO 639-1 two-character code is defined in the IANA registry.

   When a language has no ISO 639-1 two-character code and the ISO
   639-2/T (Terminology) code and the ISO 639-2/B (Bibliographic) code
   for that language differ, only the Terminology code is defined in the
   IANA registry.  At the time this document was created, all languages
   that had both kinds of three-character codes were also assigned a



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   two-character code; it is expected that future assignments of this
   nature will not occur.

   In order to avoid instability in the canonical form of tags, if a
   two-character code is added to ISO 639-1 for a language for which a
   three-character code was already included in either ISO 639-2 or ISO
   639-3, the two-character code MUST NOT be registered.  See
   Section 3.4.

   For example, if some content were tagged with 'haw' (Hawaiian), which
   currently has no two-character code, the tag would not need to be
   changed if ISO 639-1 were to assign a two-character code to the
   Hawaiian language at a later date.

   To avoid these problems with versioning and subtag choice (as
   experienced during the transition between RFC 1766 and RFC 3066), as
   well as to ensure the canonical nature of subtags defined by this
   document, the ISO 639 Registration Authority Joint Advisory Committee
   (ISO 639/RA-JAC) has included the following statement in
   [iso639.prin]:

      "A language code already in ISO 639-2 at the point of freezing ISO
      639-1 shall not later be added to ISO 639-1.  This is to ensure
      consistency in usage over time, since users are directed in
      Internet applications to employ the alpha-3 code when an alpha-2
      code for that language is not available."

2.2.2.  Extended Language Subtags

   Extended language subtags are used to identify certain specially
   selected languages that, for various historical and compatibility
   reasons, are closely identified with or tagged using an existing
   primary language subtag.  Extended language subtags are always used
   with their enclosing primary language subtag (indicated with a
   'Prefix' field in the registry) when used to form the language tag.
   All languages that have an extended language subtag in the registry
   also have an identical primary language subtag record in the
   registry.  This primary language subtag is RECOMMENDED for forming
   the language tag.  The following rules apply to the extended language
   subtags:

   1.  Extended language subtags consist solely of three-letter subtags.
       All extended language subtag records defined in the registry were
       defined according to the assignments found in [ISO639-3].
       Language collections and groupings, such as defined in
       [ISO639-5], are specifically excluded from being extended
       language subtags.




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   2.  Extended language subtag records MUST include exactly one
       'Prefix' field indicating an appropriate subtag or sequence of
       subtags for that extended language subtag.

   3.  Extended language subtag records MUST include a 'Preferred-
       Value'.  The 'Preferred-Value' and 'Subtag' fields MUST be
       identical.

   4.  Although the ABNF production 'extlang' permits up to three
       extended language tags in the language tag, extended language
       subtags MUST NOT include another extended language subtag in
       their 'Prefix'.  That is, the second and third extended language
       subtag positions in a language tag are permanently reserved and
       tags that include those subtags in that position are, and will
       always remain, invalid.

   For example, the macrolanguage Chinese ('zh') encompasses a number of
   languages.  For compatibility reasons, each of these languages has
   both a primary and extended language subtag in the registry.  A few
   selected examples of these include Gan Chinese ('gan'), Cantonese
   Chinese ('yue'), and Mandarin Chinese ('cmn').  Each is encompassed
   by the macrolanguage 'zh' (Chinese).  Therefore, they each have the
   prefix "zh" in their registry records.  Thus, Gan Chinese is
   represented with tags beginning "zh-gan" or "gan", Cantonese with
   tags beginning either "yue" or "zh-yue", and Mandarin Chinese with
   "zh-cmn" or "cmn".  The language subtag 'zh' can still be used
   without an extended language subtag to label a resource as some
   unspecified variety of Chinese, while the primary language subtag
   ('gan', 'yue', 'cmn') is preferred to using the extended language
   form ("zh-gan", "zh-yue", "zh-cmn").

2.2.3.  Script Subtag

   Script subtags are used to indicate the script or writing system
   variations that distinguish the written forms of a language or its
   dialects.  The following rules apply to the script subtags:

   1.  Script subtags MUST follow any primary and extended language
       subtags and MUST precede any other type of subtag.

   2.  Script subtags consist of four letters and were defined according
       to the assignments found in [ISO15924] ("Information and
       documentation -- Codes for the representation of names of
       scripts"), or subsequently assigned by the ISO 15924 registration
       authority or governing standardization bodies.  Only codes
       assigned by ISO 15924 will be considered for registration.





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   3.  The script subtags 'Qaaa' through 'Qabx' are reserved for private
       use in language tags.  These subtags correspond to codes reserved
       by ISO 15924 for private use.  These codes MAY be used for non-
       registered script values.  Please refer to Section 4.6 for more
       information on private use subtags.

   4.  There MUST be at most one script subtag in a language tag, and
       the script subtag SHOULD be omitted when it adds no
       distinguishing value to the tag or when the primary or extended
       language subtag's record in the subtag registry includes a
       'Suppress-Script' field listing the applicable script subtag.

   For example: "sr-Latn" represents Serbian written using the Latin
   script.

2.2.4.  Region Subtag

   Region subtags are used to indicate linguistic variations associated
   with or appropriate to a specific country, territory, or region.
   Typically, a region subtag is used to indicate variations such as
   regional dialects or usage, or region-specific spelling conventions.
   It can also be used to indicate that content is expressed in a way
   that is appropriate for use throughout a region, for instance,
   Spanish content tailored to be useful throughout Latin America.

   The following rules apply to the region subtags:

   1.  Region subtags MUST follow any primary language, extended
       language, or script subtags and MUST precede any other type of
       subtag.

   2.  Two-letter region subtags were defined according to the
       assignments found in [ISO3166-1] ("Codes for the representation
       of names of countries and their subdivisions -- Part 1: Country
       codes"), using the list of alpha-2 country codes or using
       assignments subsequently made by the ISO 3166-1 maintenance
       agency or governing standardization bodies.  In addition, the
       codes that are "exceptionally reserved" (as opposed to
       "assigned") in ISO 3166-1 were also defined in the registry, with
       the exception of 'UK', which is an exact synonym for the assigned
       code 'GB'.

   3.  The region subtags 'AA', 'QM'-'QZ', 'XA'-'XZ', and 'ZZ' are
       reserved for private use in language tags.  These subtags
       correspond to codes reserved by ISO 3166 for private use.  These
       codes MAY be used for private use region subtags (instead of
       using a private use subtag sequence).  Please refer to
       Section 4.6 for more information on private use subtags.



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   4.  Three-character region subtags consist solely of digit (number)
       characters and were defined according to the assignments found in
       the UN Standard Country or Area Codes for Statistical  Use
       [UN_M.49] or assignments subsequently made by the governing
       standards body.  Not all of the UN M.49 codes are defined in the
       IANA registry.  The following rules define which codes are
       entered into the registry as valid subtags:

       A.  UN numeric codes assigned to 'macro-geographical
           (continental)' or sub-regions MUST be registered in the
           registry.  These codes are not associated with an assigned
           ISO 3166-1 alpha-2 code and represent supra-national areas,
           usually covering more than one nation, state, province, or
           territory.

       B.  UN numeric codes for 'economic groupings' or 'other
           groupings' MUST NOT be registered in the IANA registry and
           MUST NOT be used to form language tags.

       C.  When ISO 3166-1 reassigns a code formerly used for one
           country or area to another country or area and that code
           already is present in the registry, the UN numeric code for
           that country or area MUST be registered in the registry as
           described in Section 3.4 and MUST be used to form language
           tags that represent the country or region for which it is
           defined (rather than the recycled ISO 3166-1 code).

       D.  UN numeric codes for countries or areas for which there is an
           associated ISO 3166-1 alpha-2 code in the registry MUST NOT
           be entered into the registry and MUST NOT be used to form
           language tags.  Note that the ISO 3166-based subtag in the
           registry MUST actually be associated with the UN M.49 code in
           question.

       E.  For historical reasons, the UN numeric code 830 (Channel
           Islands), which was not registered at the time this document
           was adopted and had, at that time, no corresponding ISO
           3166-1 code, MAY be entered into the IANA registry via the
           process described in Section 3.5, provided no ISO 3166-1 code
           with that exact meaning has been previously registered.

       F.  All other UN numeric codes for countries or areas that do not
           have an associated ISO 3166-1 alpha-2 code MUST NOT be
           entered into the registry and MUST NOT be used to form
           language tags.  For more information about these codes, see
           Section 3.4.





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   5.  The alphanumeric codes in Appendix X of the UN document MUST NOT
       be entered into the registry and MUST NOT be used to form
       language tags.  (At the time this document was created, these
       values matched the ISO 3166-1 alpha-2 codes.)

   6.  There MUST be at most one region subtag in a language tag and the
       region subtag MAY be omitted, as when it adds no distinguishing
       value to the tag.

   For example:

      "de-AT" represents German ('de') as used in Austria ('AT').

      "sr-Latn-RS" represents Serbian ('sr') written using Latin script
      ('Latn') as used in Serbia ('RS').

      "es-419" represents Spanish ('es') appropriate to the UN-defined
      Latin America and Caribbean region ('419').

2.2.5.  Variant Subtags

   Variant subtags are used to indicate additional, well-recognized
   variations that define a language or its dialects that are not
   covered by other available subtags.  The following rules apply to the
   variant subtags:

   1.  Variant subtags MUST follow any primary language, extended
       language, script, or region subtags and MUST precede any
       extension or private use subtag sequences.

   2.  Variant subtags, as a collection, are not associated with any
       particular external standard.  The meaning of variant subtags in
       the registry is defined in the course of the registration process
       defined in Section 3.5.  Note that any particular variant subtag
       might be associated with some external standard.  However,
       association with a standard is not required for registration.

   3.  More than one variant MAY be used to form the language tag.

   4.  Variant subtags MUST be registered with IANA according to the
       rules in Section 3.5 of this document before being used to form
       language tags.  In order to distinguish variants from other types
       of subtags, registrations MUST meet the following length and
       content restrictions:

       1.  Variant subtags that begin with a letter (a-z, A-Z) MUST be
           at least five characters long.




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       2.  Variant subtags that begin with a digit (0-9) MUST be at
           least four characters long.

   5.  The same variant subtag MUST NOT be used more than once within a
       language tag.

       *  For example, the tag "de-DE-1901-1901" is not valid.

   Variant subtag records in the Language Subtag Registry MAY include
   one or more 'Prefix' (Section 3.1.8) fields.  Each 'Prefix' indicates
   a suitable sequence of subtags for forming (with other subtags, as
   appropriate) a language tag when using the variant.

   Most variants that share a prefix are mutually exclusive.  For
   example, the German orthographic variations '1996' and '1901' SHOULD
   NOT be used in the same tag, as they represent the dates of different
   spelling reforms.  A variant that can meaningfully be used in
   combination with another variant SHOULD include a 'Prefix' field in
   its registry record that lists that other variant.  For example, if
   another German variant 'example' were created that made sense to use
   with '1996', then 'example' should include two 'Prefix' fields: "de"
   and "de-1996".

   For example:

      "sl-nedis" represents the Natisone or Nadiza dialect of Slovenian.

      "de-CH-1996" represents German as used in Switzerland and as
      written using the spelling reform beginning in the year 1996 C.E.

2.2.6.  Extension Subtags

   Extensions provide a mechanism for extending language tags for use in
   various applications.  They are intended to identify information that
   is commonly used in association with languages or language tags but
   that is not part of language identification.  See Section 3.7.  The
   following rules apply to extensions:

   1.  An extension MUST follow at least a primary language subtag.
       That is, a language tag cannot begin with an extension.
       Extensions extend language tags, they do not override or replace
       them.  For example, "a-value" is not a well-formed language tag,
       while "de-a-value" is.  Note that extensions cannot be used in
       tags that are entirely private use (that is, tags starting with
       "x-").






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   2.  Extension subtags are separated from the other subtags defined in
       this document by a single-character subtag (called a
       "singleton").  The singleton MUST be one allocated to a
       registration authority via the mechanism described in Section 3.7
       and MUST NOT be the letter 'x', which is reserved for private use
       subtag sequences.

   3.  Each singleton subtag MUST appear at most one time in each tag
       (other than as a private use subtag).  That is, singleton subtags
       MUST NOT be repeated.  For example, the tag "en-a-bbb-a-ccc" is
       invalid because the subtag 'a' appears twice.  Note that the tag
       "en-a-bbb-x-a-ccc" is valid because the second appearance of the
       singleton 'a' is in a private use sequence.

   4.  Extension subtags MUST meet whatever requirements are set by the
       document that defines their singleton prefix and whatever
       requirements are provided by the maintaining authority.  Note
       that there might not be a registry of these subtags and
       validating processors are not required to validate extensions.

   5.  Each extension subtag MUST be from two to eight characters long
       and consist solely of letters or digits, with each subtag
       separated by a single '-'.  Case distinctions are ignored in
       extensions (as with any language subtag) and normalized subtags
       of this type are expected to be in lowercase.

   6.  Each singleton MUST be followed by at least one extension subtag.
       For example, the tag "tlh-a-b-foo" is invalid because the first
       singleton 'a' is followed immediately by another singleton 'b'.

   7.  Extension subtags MUST follow all primary language, extended
       language, script, region, and variant subtags in a tag and MUST
       precede any private use subtag sequences.

   8.  All subtags following the singleton and before another singleton
       are part of the extension.  Example: In the tag "fr-a-Latn", the
       subtag 'Latn' does not represent the script subtag 'Latn' defined
       in the IANA Language Subtag Registry.  Its meaning is defined by
       the extension 'a'.

   9.  In the event that more than one extension appears in a single
       tag, the tag SHOULD be canonicalized as described in Section 4.5,
       by ordering the various extension sequences into case-insensitive
       ASCII order.

   For example, if an extension were defined for the singleton 'r' and
   it defined the subtags shown, then the following tag would be a valid
   example: "en-Latn-GB-boont-r-extended-sequence-x-private".



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2.2.7.  Private Use Subtags

   Private use subtags are used to indicate distinctions in language
   that are important in a given context by private agreement.  The
   following rules apply to private use subtags:

   1.  Private use subtags are separated from the other subtags defined
       in this document by the reserved single-character subtag 'x'.

   2.  Private use subtags MUST conform to the format and content
       constraints defined in the ABNF for all subtags; that is, they
       MUST consist solely of letters and digits and not exceed eight
       characters in length.

   3.  Private use subtags MUST follow all primary language, extended
       language, script, region, variant, and extension subtags in the
       tag.  Another way of saying this is that all subtags following
       the singleton 'x' MUST be considered private use.  Example: The
       subtag 'US' in the tag "en-x-US" is a private use subtag.

   4.  A tag MAY consist entirely of private use subtags.

   5.  No source is defined for private use subtags.  Use of private use
       subtags is by private agreement only.

   6.  Private use subtags are NOT RECOMMENDED where alternatives exist
       or for general interchange.  See Section 4.6 for more information
       on private use subtag choice.

   For example, suppose a group of scholars is studying some texts in
   medieval Greek.  They might agree to use some collection of private
   use subtags to identify different styles of writing in the texts.
   For example, they might use 'el-x-koine' for documents in the
   "common" style while using 'el-x-attic' for other documents that
   mimic the Attic style.  These subtags would not be recognized by
   outside processes or systems, but might be useful in categorizing
   various texts for study by those in the group.

   In the registry, there are also subtags derived from codes reserved
   by ISO 639, ISO 15924, or ISO 3166 for private use.  Do not confuse
   these with private use subtag sequences following the subtag 'x'.
   See Section 4.6.

2.2.8.  Grandfathered and Redundant Registrations

   Prior to RFC 4646, whole language tags were registered according to
   the rules in RFC 1766 and/or RFC 3066.  All of these registered tags
   remain valid as language tags.



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   Many of these registered tags were made redundant by the advent of
   either RFC 4646 or this document.  A redundant tag is a grandfathered
   registration whose individual subtags appear with the same semantic
   meaning in the registry.  For example, the tag "zh-Hant" (Traditional
   Chinese) can now be composed from the subtags 'zh' (Chinese) and
   'Hant' (Han script traditional variant).  These redundant tags are
   maintained in the registry as records of type 'redundant', mostly as
   a matter of historical curiosity.

   The remainder of the previously registered tags are "grandfathered".
   These tags are classified into two groups: 'regular' and 'irregular'.

   Grandfathered tags that (appear to) match the 'langtag' production in
   Figure 1 are considered 'regular' grandfathered tags.  These tags
   contain one or more subtags that either do not individually appear in
   the registry or appear but with a different semantic meaning: each
   tag, in its entirety, represents a language or collection of
   languages.

   Grandfathered tags that do not match the 'langtag' production in the
   ABNF and would otherwise be invalid are considered 'irregular'
   grandfathered tags.  With the exception of "en-GB-oed", which is a
   variant of "en-GB", each of them, in its entirety, represents a
   language.

   Many of the grandfathered tags have been superseded by the subsequent
   addition of new subtags: each superseded record contains a
   'Preferred-Value' field that ought to be used to form language tags
   representing that value.  For example, the tag "art-lojban" is
   superseded by the primary language subtag 'jbo'.

2.2.9.  Classes of Conformance

   Implementations sometimes need to describe their capabilities with
   regard to the rules and practices described in this document.  Tags
   can be checked or verified in a number of ways, but two particular
   classes of tag conformance are formally defined here.

   A tag is considered "well-formed" if it conforms to the ABNF
   (Section 2.1).  Language tags may be well-formed in terms of syntax
   but not valid in terms of content.  However, many operations
   involving language tags work well without knowing anything about the
   meaning or validity of the subtags.

   A tag is considered "valid" if it satisfies these conditions:

   o  The tag is well-formed.




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   o  Either the tag is in the list of grandfathered tags or all of its
      primary language, extended language, script, region, and variant
      subtags appear in the IANA Language Subtag Registry as of the
      particular registry date.

   o  There are no duplicate variant subtags.

   o  There are no duplicate singleton (extension) subtags.

   Note that a tag's validity depends on the date of the registry used
   to validate the tag.  A more recent copy of the registry might
   contain a subtag that an older version does not.

   A tag is considered valid for a given extension (Section 3.7) (as of
   a particular version, revision, and date) if it meets the criteria
   for "valid" above and also satisfies this condition:

      Each subtag used in the extension part of the tag is valid
      according to the extension.

   Older specifications or language tag implementations sometimes
   reference [RFC3066].  A wider array of tags was considered well-
   formed under that document.  Any tags that were valid for use under
   RFC 3066 are both well-formed and valid under this document's syntax;
   only invalid or illegal tags were well-formed under the earlier
   definition but no longer are.  The language tag syntax under RFC 3066
   was:

       obs-language-tag = primary-subtag *( "-" subtag )
       primary-subtag   = 1*8ALPHA
       subtag           = 1*8(ALPHA / DIGIT)

                  Figure 2: RFC 3066 Language Tag Syntax

   Subtags designated for private use as well as private use sequences
   introduced by the 'x' subtag are available for cases in which no
   assigned subtags are available and registration is not a suitable
   option.  For example, one might use a tag such as "no-QQ", where 'QQ'
   is one of a range of private use ISO 3166-1 codes to indicate an
   otherwise undefined region.  Users MUST NOT assign language tags that
   use subtags that do not appear in the registry other than in private
   use sequences (such as the subtag 'personal' in the tag "en-x-
   personal").  Besides not being valid, the user also risks collision
   with a future possible assignment or registrations.

   Note well: although the 'Language-Tag' production appearing in this
   document is functionally equivalent to the one in [RFC4646], it has




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   been changed to prevent certain errors in well-formedness arising
   from the old 'grandfathered' production.

3.  Registry Format and Maintenance

   The IANA Language Subtag Registry ("the registry") contains a
   comprehensive list of all of the subtags valid in language tags.
   This allows implementers a straightforward and reliable way to
   validate language tags.  The registry will be maintained so that,
   except for extension subtags, it is possible to validate all of the
   subtags that appear in a language tag under the provisions of this
   document or its revisions or successors.  In addition, the meaning of
   the various subtags will be unambiguous and stable over time.  (The
   meaning of private use subtags, of course, is not defined by the
   registry.)

   This section defines the registry along with the maintenance and
   update procedures associated with it, as well as a registry for
   extensions to language tags (Section 3.7).

3.1.  Format of the IANA Language Subtag Registry

   The IANA Language Subtag Registry is a machine-readable file in the
   format described in this section, plus copies of the registration
   forms approved in accordance with the process described in
   Section 3.5.

   The existing registration forms for grandfathered and redundant tags
   taken from RFC 3066 have been maintained as part of the obsolete RFC
   3066 registry.  The subtags added to the registry by either [RFC4645]
   or [RFC5645] do not have separate registration forms (so no forms are
   archived for these additions).

3.1.1.  File Format

   The registry is a [Unicode] text file and consists of a series of
   records in a format based on "record-jar" (described in
   [record-jar]).  Each record, in turn, consists of a series of fields
   that describe the various subtags and tags.  The actual registry file
   is encoded using the UTF-8 [RFC3629] character encoding.

   Each field can be considered a single, logical line of characters.
   Each field contains a "field-name" and a "field-body".  These are
   separated by a "field-separator".  The field-separator is a COLON
   character (U+003A) plus any surrounding whitespace.  Each field is
   terminated by the newline sequence CRLF.  The text in each field MUST
   be in Unicode Normalization Form C (NFC).




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   A collection of fields forms a "record".  Records are separated by
   lines containing only the sequence "%%" (U+0025 U+0025).

   Although fields are logically a single line of text, each line of
   text in the file format is limited to 72 bytes in length.  To
   accommodate this, the field-body can be split into a multiple-line
   representation; this is called "folding".  Folding is done according
   to customary conventions for line-wrapping.  This is typically on
   whitespace boundaries, but can occur between other characters when
   the value does not include spaces, such as when a language does not
   use whitespace between words.  In any event, there MUST NOT be breaks
   inside a multibyte UTF-8 sequence or in the middle of a combining
   character sequence.  For more information, see [UAX14].

   Although the file format uses the Unicode character set and the file
   itself is encoded using the UTF-8 encoding, fields are restricted to
   the printable characters from the US-ASCII [ISO646] repertoire unless
   otherwise indicated in the description of a specific field
   (Section 3.1.2).

   The format of the registry is described by the following ABNF
   [RFC5234].  Character numbers (code points) are taken from Unicode,
   and terminals in the ABNF productions are in terms of characters
   rather than bytes.

   registry   = record *("%%" CRLF record)
   record     = 1*field
   field      = ( field-name field-sep field-body CRLF )
   field-name = (ALPHA / DIGIT) [*(ALPHA / DIGIT / "-") (ALPHA / DIGIT)]
   field-sep  = *SP ":" *SP
   field-body = *([[*SP CRLF] 1*SP] 1*CHARS)
   CHARS      = (%x21-10FFFF)      ; Unicode code points

                      Figure 3: Registry Format ABNF

   The sequence '..'  (U+002E U+002E) in a field-body denotes a range of
   values.  Such a range represents all subtags of the same length that
   are in alphabetic or numeric order within that range, including the
   values explicitly mentioned.  For example, 'a..c' denotes the values
   'a', 'b', and 'c', and '11..13' denotes the values '11', '12', and
   '13'.

   All fields whose field-body contains a date value use the "full-date"
   format specified in [RFC3339].  For example, "2004-06-28" represents
   June 28, 2004, in the Gregorian calendar.






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3.1.2.  Record and Field Definitions

   There are three types of records in the registry: "File-Date",
   "Subtag", and "Tag".

   The first record in the registry is always the "File-Date" record.
   This record occurs only once in the file and contains a single field
   whose field-name is "File-Date".  The field-body of this record
   contains a date (see Section 5.1), making it possible to easily
   recognize different versions of the registry.

   File-Date: 2004-06-28
   %%

                 Figure 4: Example of the File-Date Record

   Subsequent records contain multiple fields and represent information
   about either subtags or tags.  Both types of records have an
   identical structure, except that "Subtag" records contain a field
   with a field-name of "Subtag", while, unsurprisingly, "Tag" records
   contain a field with a field-name of "Tag".  Field-names MUST NOT
   occur more than once per record, with the exception of the
   'Description', 'Comments', and 'Prefix' fields.

   Each record MUST contain at least one of each of the following
   fields:

   o  'Type'

      *  Type's field-body MUST consist of one of the following strings:
         "language", "extlang", "script", "region", "variant",
         "grandfathered", and "redundant"; it denotes the type of tag or
         subtag.

   o  Either 'Subtag' or 'Tag'

      *  Subtag's field-body contains the subtag being defined.  This
         field MUST appear in all records whose 'Type' has one of these
         values: "language", "extlang", "script", "region", or
         "variant".

      *  Tag's field-body contains a complete language tag.  This field
         MUST appear in all records whose 'Type' has one of these
         values: "grandfathered" or "redundant".  If the 'Type' is
         "grandfathered", then the 'Tag' field-body will be one of the
         tags listed in either the 'regular' or 'irregular' production
         found in Section 2.1.




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   o  'Description'

      *  Description's field-body contains a non-normative description
         of the subtag or tag.

   o  'Added'

      *  Added's field-body contains the date the record was registered
         or, in the case of grandfathered or redundant tags, the date
         the corresponding tag was registered under the rules of
         [RFC1766] or [RFC3066].

   Each record MAY also contain the following fields:

   o  'Deprecated'

      *  Deprecated's field-body contains the date the record was
         deprecated.  In some cases, this value is earlier than that of
         the 'Added' field in the same record.  That is, the date of
         deprecation preceded the addition of the record to the
         registry.

   o  'Preferred-Value'

      *  Preferred-Value's field-body contains a canonical mapping from
         this record's value to a modern equivalent that is preferred in
         its place.  Depending on the value of the 'Type' field, this
         value can take different forms:

         +  For fields of type 'language', 'Preferred-Value' contains
            the primary language subtag that is preferred when forming
            the language tag.

         +  For fields of type 'script', 'region', or 'variant',
            'Preferred-Value' contains the subtag of the same type that
            is preferred for forming the language tag.

         +  For fields of type 'extlang', 'grandfathered', or
            'redundant', 'Preferred-Value' contains an "extended
            language range" [RFC4647] that is preferred for forming the
            language tag.  That is, the preferred language tag will
            contain, in order, each of the subtags that appears in the
            'Preferred-Value'; additional fields can be included in a
            language tag, as described elsewhere in this document.  For
            example, the replacement for the grandfathered tag "zh-min-
            nan" (Min Nan Chinese) is "nan", which can be used as the





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            basis for tags such as "nan-Hant" or "nan-TW" (note that the
            extended language subtag form such as "zh-nan-Hant" or "zh-
            nan-TW" can also be used).

   o  'Prefix'

      *  Prefix's field-body contains a valid language tag that is
         RECOMMENDED as one possible prefix to this record's subtag.
         This field MAY appear in records whose 'Type' field-body is
         either 'extlang' or 'variant' (it MUST NOT appear in any other
         record type).

   o  'Suppress-Script'

      *  Suppress-Script's field-body contains a script subtag that
         SHOULD NOT be used to form language tags with the associated
         primary or extended language subtag.  This field MUST appear
         only in records whose 'Type' field-body is 'language' or
         'extlang'.  See Section 4.1.

   o  'Macrolanguage'

      *  Macrolanguage's field-body contains a primary language subtag
         defined by ISO 639 as the "macrolanguage" that encompasses this
         language subtag.  This field MUST appear only in records whose
         'Type' field-body is either 'language' or 'extlang'.

   o  'Scope'

      *  Scope's field-body contains information about a primary or
         extended language subtag indicating the type of language code
         according to ISO 639.  The values permitted in this field are
         "macrolanguage", "collection", "special", and "private-use".
         This field only appears in records whose 'Type' field-body is
         either 'language' or 'extlang'.  When this field is omitted,
         the language is an individual language.

   o  'Comments'

      *  Comments's field-body contains additional information about the
         subtag, as deemed appropriate for understanding the registry
         and implementing language tags using the subtag or tag.

   Future versions of this document might add additional fields to the
   registry; implementations SHOULD ignore fields found in the registry
   that are not defined in this document.





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3.1.3.  Type Field

   The field 'Type' contains the string identifying the record type in
   which it appears.  Values for the 'Type' field-body are: "language"
   (Section 2.2.1); "extlang" (Section 2.2.2); "script" (Section 2.2.3);
   "region" (Section 2.2.4); "variant" (Section 2.2.5); "grandfathered"
   or "redundant" (Section 2.2.8).

3.1.4.  Subtag and Tag Fields

   The field 'Subtag' contains the subtag defined in the record.  The
   field 'Tag' appears in records whose 'Type' is either 'grandfathered'
   or 'redundant' and contains a tag registered under [RFC3066].

   The 'Subtag' field-body MUST follow the casing conventions described
   in Section 2.1.1.  All subtags use lowercase letters in the field-
   body, with two exceptions:

      Subtags whose 'Type' field is 'script' (in other words, subtags
      defined by ISO 15924) MUST use titlecase.

      Subtags whose 'Type' field is 'region' (in other words, the non-
      numeric region subtags defined by ISO 3166-1) MUST use all
      uppercase.

   The 'Tag' field-body MUST be formatted according to the rules
   described in Section 2.1.1.

3.1.5.  Description Field

   The field 'Description' contains a description of the tag or subtag
   in the record.  The 'Description' field MAY appear more than once per
   record.  The 'Description' field MAY include the full range of
   Unicode characters.  At least one of the 'Description' fields MUST be
   written or transcribed into the Latin script; additional
   'Description' fields MAY be in any script or language.

   The 'Description' field is used for identification purposes.
   Descriptions SHOULD contain all and only that information necessary
   to distinguish one subtag from others with which it might be
   confused.  They are not intended to provide general background
   information or to provide all possible alternate names or
   designations.  'Description' fields don't necessarily represent the
   actual native name of the item in the record, nor are any of the
   descriptions guaranteed to be in any particular language (such as
   English or French, for example).





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   Descriptions in the registry that correspond to ISO 639, ISO 15924,
   ISO 3166-1, or UN M.49 codes are intended only to indicate the
   meaning of that identifier as defined in the source standard at the
   time it was added to the registry or as subsequently modified, within
   the bounds of the stability rules (Section 3.4), via subsequent
   registration.  The 'Description' does not replace the content of the
   source standard itself.  'Description' fields are not intended to be
   the localized English names for the subtags.  Localization or
   translation of language tag and subtag descriptions is out of scope
   of this document.

   For subtags taken from a source standard (such as ISO 639 or ISO
   15924), the 'Description' fields in the record are also initially
   taken from that source standard.  Multiple descriptions in the source
   standard are split into separate 'Description' fields.  The source
   standard's descriptions MAY be edited or modified, either prior to
   insertion or via the registration process, and additional or
   extraneous descriptions omitted or removed.  Each 'Description' field
   MUST be unique within the record in which it appears, and formatting
   variations of the same description SHOULD NOT occur in that specific
   record.  For example, while the ISO 639-1 code 'fy' has both the
   description "Western Frisian" and the description "Frisian, Western"
   in that standard, only one of these descriptions appears in the
   registry.

   To help ensure that users do not become confused about which subtag
   to use, 'Description' fields assigned to a record of any specific
   type ('language', 'extlang', 'script', and so on) MUST be unique
   within that given record type with the following exception: if a
   particular 'Description' field occurs in multiple records of a given
   type, then at most one of the records can omit the 'Deprecated'
   field.  All deprecated records that share a 'Description' MUST have
   the same 'Preferred-Value', and all non-deprecated records MUST be
   that 'Preferred-Value'.  This means that two records of the same type
   that share a 'Description' are also semantically equivalent and no
   more than one record with a given 'Description' is preferred for that
   meaning.

   For example, consider the 'language' subtags 'zza' (Zaza) and 'diq'
   (Dimli).  It so happens that 'zza' is a macrolanguage enclosing 'diq'
   and thus also has a description in ISO 639-3 of "Dimli".  This
   description was edited to read "Dimli (macrolanguage)" in the
   registry record for 'zza' to prevent a collision.

   By contrast, the subtags 'he' and 'iw' share a 'Description' value of
   "Hebrew"; this is permitted because 'iw' is deprecated and its
   'Preferred-Value' is 'he'.




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   For fields of type 'language', the first 'Description' field
   appearing in the registry corresponds whenever possible to the
   Reference Name assigned by ISO 639-3.  This helps facilitate cross-
   referencing between ISO 639 and the registry.

   When creating or updating a record due to the action of one of the
   source standards, the Language Subtag Reviewer MAY edit descriptions
   to correct irregularities in formatting (such as misspellings,
   inappropriate apostrophes or other punctuation, or excessive or
   missing spaces) prior to submitting the proposed record to the
   ietf-languages@iana.org list for consideration.

3.1.6.  Deprecated Field

   The field 'Deprecated' contains the date the record was deprecated
   and MAY be added, changed, or removed from any record via the
   maintenance process described in Section 3.3 or via the registration
   process described in Section 3.5.  Usually, the addition of a
   'Deprecated' field is due to the action of one of the standards
   bodies, such as ISO 3166, withdrawing a code.  Although valid in
   language tags, subtags and tags with a 'Deprecated' field are
   deprecated, and validating processors SHOULD NOT generate these
   subtags.  Note that a record that contains a 'Deprecated' field and
   no corresponding 'Preferred-Value' field has no replacement mapping.

   In some historical cases, it might not have been possible to
   reconstruct the original deprecation date.  For these cases, an
   approximate date appears in the registry.  Some subtags and some
   grandfathered or redundant tags were deprecated before the initial
   creation of the registry.  The exact rules for this appear in Section
   2 of [RFC4645].  Note that these records have a 'Deprecated' field
   with an earlier date then the corresponding 'Added' field!

3.1.7.  Preferred-Value Field

   The field 'Preferred-Value' contains a mapping between the record in
   which it appears and another tag or subtag (depending on the record's
   'Type').  The value in this field is used for canonicalization (see
   Section 4.5).  In cases where the subtag or tag also has a
   'Deprecated' field, then the 'Preferred-Value' is RECOMMENDED as the
   best choice to represent the value of this record when selecting a
   language tag.

   Records containing a 'Preferred-Value' fall into one of these four
   groups:






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   1.  ISO 639 language codes that were later withdrawn in favor of
       other codes.  These values are mostly a historical curiosity.
       The 'he'/'iw' pairing above is an example of this.

   2.  Subtags (with types other than language or extlang) taken from
       codes or values that have been withdrawn in favor of a new code.
       In particular, this applies to region subtags taken from ISO
       3166-1, because sometimes a country will change its name or
       administration in such a way that warrants a new region code.  In
       some cases, countries have reverted to an older name, which might
       already be encoded.  For example, the subtag 'ZR' (Zaire) was
       replaced by the subtag 'CD' (Democratic Republic of the Congo)
       when that country's name was changed.

   3.  Tags or subtags that have become obsolete because the values they
       represent were later encoded.  Many of the grandfathered or
       redundant tags were later encoded by ISO 639, for example, and
       fall into this grouping.  For example, "i-klingon" was deprecated
       when the subtag 'tlh' was added.  The record for "i-klingon" has
       a 'Preferred-Value' of 'tlh'.

   4.  Extended language subtags always have a mapping to their
       identical primary language subtag.  For example, the extended
       language subtag 'yue' (Cantonese) can be used to form the tag
       "zh-yue".  It has a 'Preferred-Value' mapping to the primary
       language subtag 'yue', meaning that a tag such as
       "zh-yue-Hant-HK" can be canonicalized to "yue-Hant-HK".

   Records other than those of type 'extlang' that contain a 'Preferred-
   Value' field MUST also have a 'Deprecated' field.  This field
   contains the date on which the tag or subtag was deprecated in favor
   of the preferred value.

   For records of type 'extlang', the 'Preferred-Value' field appears
   without a corresponding 'Deprecated' field.  An implementation MAY
   ignore these preferred value mappings, although if it ignores the
   mapping, it SHOULD do so consistently.  It SHOULD also treat the
   'Preferred-Value' as equivalent to the mapped item.  For example, the
   tags "zh-yue-Hant-HK" and "yue-Hant-HK" are semantically equivalent
   and ought to be treated as if they were the same tag.

   Occasionally, the deprecated code is preferred in certain contexts.
   For example, both "iw" and "he" can be used in the Java programming
   language, but "he" is converted on input to "iw", which is thus the
   canonical form in Java.






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   'Preferred-Value' mappings in records of type 'region' sometimes do
   not represent exactly the same meaning as the original value.  There
   are many reasons for a country code to be changed, and the effect
   this has on the formation of language tags will depend on the nature
   of the change in question.  For example, the region subtag 'YD'
   (Democratic Yemen) was deprecated in favor of the subtag 'YE' (Yemen)
   when those two countries unified in 1990.

   A 'Preferred-Value' MAY be added to, changed, or removed from records
   according to the rules in Section 3.3.  Addition, modification, or
   removal of a 'Preferred-Value' field in a record does not imply that
   content using the affected subtag needs to be retagged.

   The 'Preferred-Value' fields in records of type "grandfathered" and
   "redundant" each contain an "extended language range" [RFC4647] that
   is strongly RECOMMENDED for use in place of the record's value.  In
   many cases, these mappings were created via deprecation of the tags
   during the period before [RFC4646] was adopted.  For example, the tag
   "no-nyn" was deprecated in favor of the ISO 639-1-defined language
   code 'nn'.

   The 'Preferred-Value' field in subtag records of type "extlang" also
   contains an "extended language range".  This allows the subtag to be
   deprecated in favor of either a single primary language subtag or a
   new language-extlang sequence.

   Usually, the addition, removal, or change of a 'Preferred-Value'
   field for a subtag is done to reflect changes in one of the source
   standards.  For example, if an ISO 3166-1 region code is deprecated
   in favor of another code, that SHOULD result in the addition of a
   'Preferred-Value' field.

   Changes to one subtag can affect other subtags as well: when
   proposing changes to the registry, the Language Subtag Reviewer MUST
   review the registry for such effects and propose the necessary
   changes using the process in Section 3.5, although anyone MAY request
   such changes.  For example:

      Suppose that subtag 'XX' has a 'Preferred-Value' of 'YY'.  If 'YY'
      later changes to have a 'Preferred-Value' of 'ZZ', then the
      'Preferred-Value' for 'XX' MUST also change to be 'ZZ'.

      Suppose that a registered language subtag 'dialect' represents a
      language not yet available in any part of ISO 639.  The later
      addition of a corresponding language code in ISO 639 SHOULD result
      in the addition of a 'Preferred-Value' for 'dialect'.





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3.1.8.  Prefix Field

   The field 'Prefix' contains a valid language tag that is RECOMMENDED
   as one possible prefix to this record's subtag, perhaps with other
   subtags.  That is, when including an extended language or a variant
   subtag that has at least one 'Prefix' in a language tag, the
   resulting tag SHOULD match at least one of the subtag's 'Prefix'
   fields using the "Extended Filtering" algorithm (see [RFC4647]), and
   each of the subtags in that 'Prefix' SHOULD appear before the subtag
   itself.

   The 'Prefix' field MUST appear exactly once in a record of type
   'extlang'.  The 'Prefix' field MAY appear multiple times (or not at
   all) in records of type 'variant'.  Additional fields of this type
   MAY be added to a 'variant' record via the registration process,
   provided the 'variant' record already has at least one 'Prefix'
   field.

   Each 'Prefix' field indicates a particular sequence of subtags that
   form a meaningful tag with this subtag.  For example, the extended
   language subtag 'cmn' (Mandarin Chinese) only makes sense with its
   prefix 'zh' (Chinese).  Similarly, 'rozaj' (Resian, a dialect of
   Slovenian) would be appropriate when used with its prefix 'sl'
   (Slovenian), while tags such as "is-1994" are not appropriate (and
   probably not meaningful).  Although the 'Prefix' for 'rozaj' is "sl",
   other subtags might appear between them.  For example, the tag "sl-
   IT-rozaj" (Slovenian, Italy, Resian) matches the 'Prefix' "sl".

   The 'Prefix' also indicates when variant subtags make sense when used
   together (many that otherwise share a 'Prefix' are mutually
   exclusive) and what the relative ordering of variants is supposed to
   be.  For example, the variant '1994' (Standardized Resian
   orthography) has several 'Prefix' fields in the registry ("sl-rozaj",
   "sl-rozaj-biske", "sl-rozaj-njiva", "sl-rozaj-osojs", and "sl-rozaj-
   solba").  This indicates not only that '1994' is appropriate to use
   with each of these five Resian variant subtags ('rozaj', 'biske',
   'njiva', 'osojs', and 'solba'), but also that it SHOULD appear
   following any of these variants in a tag.  Thus, the language tag
   ought to take the form "sl-rozaj-biske-1994", rather than "sl-1994-
   rozaj-biske" or "sl-rozaj-1994-biske".

   If a record includes no 'Prefix' field, a 'Prefix' field MUST NOT be
   added to the record at a later date.  Otherwise, changes (additions,
   deletions, or modifications) to the set of 'Prefix' fields MAY be
   registered, as long as they strictly widen the range of language tags
   that are recommended.  For example, a 'Prefix' with the value "be-
   Latn" (Belarusian, Latin script) could be replaced by the value "be"
   (Belarusian) but not by the value "ru-Latn" (Russian, Latin script)



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   or the value "be-Latn-BY" (Belarusian, Latin script, Belarus), since
   these latter either change or narrow the range of suggested tags.

   The field-body of the 'Prefix' field MUST NOT conflict with any
   'Prefix' already registered for a given record.  Such a conflict
   would occur when no valid tag could be constructed that would contain
   the prefix, such as when two subtags each have a 'Prefix' that
   contains the other subtag.  For example, suppose that the subtag
   'avariant' has the prefix "es-bvariant".  Then the subtag 'bvariant'
   cannot be assigned the prefix 'avariant', for that would require a
   tag of the form "es-avariant-bvariant-avariant", which would not be
   valid.

3.1.9.  Suppress-Script Field

   The field 'Suppress-Script' contains a script subtag (whose record
   appears in the registry).  The field 'Suppress-Script' MUST appear
   only in records whose 'Type' field-body is either 'language' or
   'extlang'.  This field MUST NOT appear more than one time in a
   record.

   This field indicates a script used to write the overwhelming majority
   of documents for the given language.  The subtag for such a script
   therefore adds no distinguishing information to a language tag and
   thus SHOULD NOT be used for most documents in that language.
   Omitting the script subtag indicated by this field helps ensure
   greater compatibility between the language tags generated according
   to the rules in this document and language tags and tag processors or
   consumers based on RFC 3066.  For example, virtually all Icelandic
   documents are written in the Latin script, making the subtag 'Latn'
   redundant in the tag "is-Latn".

   Many language subtag records do not have a 'Suppress-Script' field.
   The lack of a 'Suppress-Script' might indicate that the language is
   customarily written in more than one script or that the language is
   not customarily written at all.  It might also mean that sufficient
   information was not available when the record was created and thus
   remains a candidate for future registration.

3.1.10.  Macrolanguage Field

   The field 'Macrolanguage' contains a primary language subtag (whose
   record appears in the registry).  This field indicates a language
   that encompasses this subtag's language according to assignments made
   by ISO 639-3.

   ISO 639-3 labels some languages in the registry as "macrolanguages".
   ISO 639-3 defines the term "macrolanguage" to mean "clusters of



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   closely-related language varieties that [...] can be considered
   distinct individual languages, yet in certain usage contexts a single
   language identity for all is needed".  These correspond to codes
   registered in ISO 639-2 as individual languages that were found to
   correspond to more than one language in ISO 639-3.

   A language contained within a macrolanguage is called an "encompassed
   language".  The record for each encompassed language contains a
   'Macrolanguage' field in the registry; the macrolanguages themselves
   are not specially marked.  Note that some encompassed languages have
   ISO 639-1 or ISO 639-2 codes.

   The 'Macrolanguage' field can only occur in records of type
   'language' or 'extlang'.  Only values assigned by ISO 639-3 will be
   considered for inclusion.  'Macrolanguage' fields MAY be added or
   removed via the normal registration process whenever ISO 639-3
   defines new values or withdraws old values.  Macrolanguages are
   informational, and MAY be removed or changed if ISO 639-3 changes the
   values.  For more information on the use of this field and choosing
   between macrolanguage and encompassed language subtags, see
   Section 4.1.1.

   For example, the language subtags 'nb' (Norwegian Bokmal) and 'nn'
   (Norwegian Nynorsk) each have a 'Macrolanguage' field with a value of
   'no' (Norwegian).  For more information, see Section 4.1.

3.1.11.  Scope Field

   The field 'Scope' contains classification information about a primary
   or extended language subtag derived from ISO 639.  Most languages
   have a scope of 'individual', which means that the language is not a
   macrolanguage, collection, special code, or private use.  That is, it
   is what one would normally consider to be 'a language'.  Any primary
   or extended language subtag that has no 'Scope' field is an
   individual language.

   'Scope' information can sometimes be helpful in selecting language
   tags, since it indicates the purpose or "scope" of the code
   assignment within ISO 639.  The available values are:

   o  'macrolanguage' - Indicates a macrolanguage as defined by ISO
      639-3 (see Section 3.1.10).  A macrolanguage is a cluster of
      closely related languages that are sometimes considered to be a
      single language.

   o  'collection' - Indicates a subtag that represents a collection of
      languages, typically related by some type of historical,
      geographical, or linguistic association.  Unlike a macrolanguage,



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      a collection can contain languages that are only loosely related
      and a collection cannot be used interchangeably with languages
      that belong to it.

   o  'special' - Indicates a special language code.  These are subtags
      used for identifying linguistic attributes not particularly
      associated with a concrete language.  These include codes for when
      the language is undetermined or for non-linguistic content.

   o  'private-use' - Indicates a code reserved for private use in the
      underlying standard.  Subtags with this scope can be used to
      indicate a primary language for which no ISO 639 or registered
      assignment exists.

   The 'Scope' field MAY appear in records of type 'language' or
   'extlang'.  Note that many of the prefixes for extended language
   subtags will have a 'Scope' of 'macrolanguage' (although some will
   not) and that many languages that have a 'Scope' of 'macrolanguage'
   will have extended language subtags associated with them.

   The 'Scope' field MAY be added, modified, or removed via the
   registration process, provided the change mirrors changes made by ISO
   639 to the assignment's classification.  Such a change is expected to
   be rare.

   For example, the primary language subtag 'zh' (Chinese) has a 'Scope'
   of 'macrolanguage', while its enclosed language 'nan' (Min Nan
   Chinese) has a 'Scope' of 'individual'.  The special value 'und'
   (Undetermined) has a 'Scope' of 'special'.  The ISO 639-5 collection
   'gem' (Germanic languages) has a 'Scope' of 'collection'.

3.1.12.  Comments Field

   The field 'Comments' contains additional information about the record
   and MAY appear more than once per record.  The field-body MAY include
   the full range of Unicode characters and is not restricted to any
   particular script.  This field MAY be inserted or changed via the
   registration process, and no guarantee of stability is provided.

   The content of this field is not restricted, except by the need to
   register the information, the suitability of the request, and by
   reasonable practical size limitations.  The primary reason for the
   'Comments' field is subtag identification -- to help distinguish the
   subtag from others with which it might be confused as an aid to
   usage.  Large amounts of information about the use, history, or
   general background of a subtag are frowned upon, as these generally
   belong in a registration request rather than in the registry.




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3.2.  Language Subtag Reviewer

   The Language Subtag Reviewer moderates the ietf-languages@iana.org
   mailing list, responds to requests for registration, and performs the
   other registry maintenance duties described in Section 3.3.  Only the
   Language Subtag Reviewer is permitted to request IANA to change,
   update, or add records to the Language Subtag Registry.  The Language
   Subtag Reviewer MAY delegate list moderation and other clerical
   duties as needed.

   The Language Subtag Reviewer is appointed by the IESG for an
   indefinite term, subject to removal or replacement at the IESG's
   discretion.  The IESG will solicit nominees for the position (upon
   adoption of this document or upon a vacancy) and then solicit
   feedback on the nominees' qualifications.  Qualified candidates
   should be familiar with BCP 47 and its requirements; be willing to
   fairly, responsively, and judiciously administer the registration
   process; and be suitably informed about the issues of language
   identification so that the reviewer can assess the claims and draw
   upon the contributions of language experts and subtag requesters.

   The subsequent performance or decisions of the Language Subtag
   Reviewer MAY be appealed to the IESG under the same rules as other
   IETF decisions (see [RFC2026]).  The IESG can reverse or overturn the
   decisions of the Language Subtag Reviewer, provide guidance, or take
   other appropriate actions.

3.3.  Maintenance of the Registry

   Maintenance of the registry requires that, as codes are assigned or
   withdrawn by ISO 639, ISO 15924, ISO 3166, and UN M.49, the Language
   Subtag Reviewer MUST evaluate each change and determine the
   appropriate course of action according to the rules in this document.
   Such updates follow the registration process described in
   Section 3.5.  Usually, the Language Subtag Reviewer will start the
   process for the new or updated record by filling in the registration
   form and submitting it.  If a change to one of these standards takes
   place and the Language Subtag Reviewer does not do this in a timely
   manner, then any interested party MAY submit the form.  Thereafter,
   the registration process continues normally.

   Note that some registrations affect other subtags--perhaps more than
   one--as when a region subtag is being deprecated in favor of a new
   value.  The Language Subtag Reviewer is responsible for ensuring that
   any such changes are properly registered, with each change requiring
   its own registration form.





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   The Language Subtag Reviewer MUST ensure that new subtags meet the
   requirements elsewhere in this document (and most especially in
   Section 3.4) or submit an appropriate registration form for an
   alternate subtag as described in that section.  Each individual
   subtag affected by a change MUST be sent to the
   ietf-languages@iana.org list with its own registration form and in a
   separate message.

3.4.  Stability of IANA Registry Entries

   The stability of entries and their meaning in the registry is
   critical to the long-term stability of language tags.  The rules in
   this section guarantee that a specific language tag's meaning is
   stable over time and will not change.

   These rules specifically deal with how changes to codes (including
   withdrawal and deprecation of codes) maintained by ISO 639, ISO
   15924, ISO 3166, and UN M.49 are reflected in the IANA Language
   Subtag Registry.  Assignments to the IANA Language Subtag Registry
   MUST follow the following stability rules:

   1.   Values in the fields 'Type', 'Subtag', 'Tag', and 'Added' MUST
        NOT be changed and are guaranteed to be stable over time.

   2.   Values in the fields 'Preferred-Value' and 'Deprecated' MAY be
        added, altered, or removed via the registration process.  These
        changes SHOULD be limited to changes necessary to mirror changes
        in one of the underlying standards (ISO 639, ISO 15924, ISO
        3166-1, or UN M.49) and typically alteration or removal of a
        'Preferred-Value' is limited specifically to region codes.

   3.   Values in the 'Description' field MUST NOT be changed in a way
        that would invalidate any existing tags.  The description MAY be
        broadened somewhat in scope, changed to add information, or
        adapted to the most common modern usage.  For example, countries
        occasionally change their names; a historical example of this is
        "Upper Volta" changing to "Burkina Faso".

   4.   Values in the field 'Prefix' MAY be added to existing records of
        type 'variant' via the registration process, provided the
        'variant' already has at least one 'Prefix'.  A 'Prefix' field
        SHALL NOT be registered for any 'variant' that has no existing
        'Prefix' field.  If a prefix is added to a variant record,
        'Comment' fields MAY be used to explain different usages with
        the various prefixes.






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   5.   Values in the field 'Prefix' in records of type 'variant' MAY
        also be modified, so long as the modifications broaden the set
        of prefixes.  That is, a prefix MAY be replaced by one of its
        own prefixes.  For example, the prefix "en-US" could be replaced
        by "en", but not by the prefixes "en-Latn", "fr", or "en-US-
        boont".  If one of those prefix values were needed, it would
        have to be separately registered.

   6.   Values in the field 'Prefix' in records of type 'extlang' MUST
        NOT be added, modified, or removed.

   7.   The field 'Prefix' MUST NOT be removed from any record in which
        it appears.  This field SHOULD be included in the initial
        registration of any records of type 'variant' and MUST be
        included in any records of type 'extlang'.

   8.   The field 'Comments' MAY be added, changed, modified, or removed
        via the registration process or any of the processes or
        considerations described in this section.

   9.   The field 'Suppress-Script' MAY be added or removed via the
        registration process.

   10.  The field 'Macrolanguage' MAY be added or removed via the
        registration process, but only in response to changes made by
        ISO 639.  The 'Macrolanguage' field appears whenever a language
        has a corresponding macrolanguage in ISO 639.  That is, the
        'Macrolanguage' fields in the registry exactly match those of
        ISO 639.  No other macrolanguage mappings will be considered for
        registration.

   11.  The field 'Scope' MAY be added or removed from a primary or
        extended language subtag after initial registration, and it MAY
        be modified in order to match any changes made by ISO 639.
        Changes to the 'Scope' field MUST mirror changes made by ISO
        639.  Note that primary or extended language subtags whose
        records do not contain a 'Scope' field (that is, most of them)
        are individual languages as described in Section 3.1.11.

   12.  Primary and extended language subtags (other than independently
        registered values created using the registration process) are
        created according to the assignments of the various parts of ISO
        639, as follows:

        A.  Codes assigned by ISO 639-1 that do not conflict with
            existing two-letter primary language subtags and that have
            no corresponding three-letter primary defined in the
            registry are entered into the IANA registry as new records



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            of type 'language'.  Note that languages given an ISO 639-1
            code cannot be given extended language subtags, even if
            encompassed by a macrolanguage.

        B.  Codes assigned by ISO 639-3 or ISO 639-5 that do not
            conflict with existing three-letter primary language subtags
            and that do not have ISO 639-1 codes assigned (or expected
            to be assigned) are entered into the IANA registry as new
            records of type 'language'.  Note that these two standards
            now comprise a superset of ISO 639-2 codes.  Codes that have
            a defined 'macrolanguage' mapping at the time of their
            registration MUST contain a 'Macrolanguage' field.

        C.  Codes assigned by ISO 639-3 MAY also be considered for an
            extended language subtag registration.  Note that they MUST
            be assigned a primary language subtag record of type
            'language' even when an 'extlang' record is proposed.  When
            considering extended language subtag assignment, these
            criteria apply:

            1.  If a language has a macrolanguage mapping, and that
                macrolanguage has other encompassed languages that are
                assigned extended language subtags, then the new
                language SHOULD have an 'extlang' record assigned to it
                as well.  For example, any language with a macrolanguage
                of 'zh' or 'ar' would be assigned an 'extlang' record.

            2.  'Extlang' records SHOULD NOT be created for languages if
                other languages encompassed by the macrolanguage do not
                also include 'extlang' records.  For example, if a new
                Serbo-Croatian ('sh') language were registered, it would
                not get an extlang record because other languages
                encompassed, such as Serbian ('sr'), do not include one
                in the registry.

            3.  Sign languages SHOULD have an 'extlang' record with a
                'Prefix' of 'sgn'.

            4.  'Extlang' records MUST NOT be created for items already
                in the registry.  Extended language subtags will only be
                considered at the time of initial registration.

            5.  Extended language subtag records MUST include the fields
                'Prefix' and 'Preferred-Value' with field values
                assigned as described in Section 2.2.2.

        D.  Any other codes assigned by ISO 639-2 that do not conflict
            with existing three-letter primary or extended language



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            subtags and that do not have ISO 639-1 two-letter codes
            assigned are entered into the IANA registry as new records
            of type 'language'.  This type of registration is not
            supposed to occur in the future.

   13.  Codes assigned by ISO 15924 and ISO 3166-1 that do not conflict
        with existing subtags of the associated type and whose meaning
        is not the same as an existing subtag of the same type are
        entered into the IANA registry as new records.

   14.  Codes assigned by ISO 639, ISO 15924, or ISO 3166-1 that are
        withdrawn by their respective maintenance or registration
        authority remain valid in language tags.  A 'Deprecated' field
        containing the date of withdrawal MUST be added to the record.
        If a new record of the same type is added that represents a
        replacement value, then a 'Preferred-Value' field MAY also be
        added.  The registration process MAY be used to add comments
        about the withdrawal of the code by the respective standard.

           For example: the region code 'TL' was assigned to the country
           'Timor-Leste', replacing the code 'TP' (which was assigned to
           'East Timor' when it was under administration by Portugal).
           The subtag 'TP' remains valid in language tags, but its
           record contains the 'Preferred-Value' of 'TL' and its field
           'Deprecated' contains the date the new code was assigned
           ('2004-07-06').

   15.  Codes assigned by ISO 639, ISO 15924, or ISO 3166-1 that
        conflict with existing subtags of the associated type, including
        subtags that are deprecated, MUST NOT be entered into the
        registry.  The following additional considerations apply to
        subtag values that are reassigned:

        A.  For ISO 639 codes, if the newly assigned code's meaning is
            not represented by a subtag in the IANA registry, the
            Language Subtag Reviewer, as described in Section 3.5, SHALL
            prepare a proposal for entering in the IANA registry, as
            soon as practical, a registered language subtag as an
            alternate value for the new code.  The form of the
            registered language subtag will be at the discretion of the
            Language Subtag Reviewer and MUST conform to other
            restrictions on language subtags in this document.

        B.  For all subtags whose meaning is derived from an external
            standard (that is, by ISO 639, ISO 15924, ISO 3166-1, or UN
            M.49), if a new meaning is assigned to an existing code and
            the new meaning broadens the meaning of that code, then the
            meaning for the associated subtag MAY be changed to match.



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            The meaning of a subtag MUST NOT be narrowed, however, as
            this can result in an unknown proportion of the existing
            uses of a subtag becoming invalid.  Note: the ISO 639
            registration authority (RA) has adopted a similar stability
            policy.

        C.  For ISO 15924 codes, if the newly assigned code's meaning is
            not represented by a subtag in the IANA registry, the
            Language Subtag Reviewer, as described in Section 3.5, SHALL
            prepare a proposal for entering in the IANA registry, as
            soon as practical, a registered variant subtag as an
            alternate value for the new code.  The form of the
            registered variant subtag will be at the discretion of the
            Language Subtag Reviewer and MUST conform to other
            restrictions on variant subtags in this document.

        D.  For ISO 3166-1 codes, if the newly assigned code's meaning
            is associated with the same UN M.49 code as another 'region'
            subtag, then the existing region subtag remains as the
            preferred value for that region and no new entry is created.
            A comment MAY be added to the existing region subtag
            indicating the relationship to the new ISO 3166-1 code.

        E.  For ISO 3166-1 codes, if the newly assigned code's meaning
            is associated with a UN M.49 code that is not represented by
            an existing region subtag, then the Language Subtag
            Reviewer, as described in Section 3.5, SHALL prepare a
            proposal for entering the appropriate UN M.49 country code
            as an entry in the IANA registry.

        F.  For ISO 3166-1 codes, if there is no associated UN numeric
            code, then the Language Subtag Reviewer SHALL petition the
            UN to create one.  If there is no response from the UN
            within 90 days of the request being sent, the Language
            Subtag Reviewer SHALL prepare a proposal for entering in the
            IANA registry, as soon as practical, a registered variant
            subtag as an alternate value for the new code.  The form of
            the registered variant subtag will be at the discretion of
            the Language Subtag Reviewer and MUST conform to other
            restrictions on variant subtags in this document.  This
            situation is very unlikely to ever occur.

   16.  UN M.49 has codes for both "countries and areas" (such as '276'
        for Germany) and "geographical regions and sub-regions" (such as
        '150' for Europe).  UN M.49 country or area codes for which
        there is no corresponding ISO 3166-1 code MUST NOT be
        registered, except as a surrogate for an ISO 3166-1 code that is
        blocked from registration by an existing subtag.



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        If such a code becomes necessary, then the maintenance agency
        for ISO 3166-1 SHALL first be petitioned to assign a code to the
        region.  If the petition for a code assignment by ISO 3166-1 is
        refused or not acted on in a timely manner, the registration
        process described in Section 3.5 can then be used to register
        the corresponding UN M.49 code.  This way, UN M.49 codes remain
        available as the value of last resort in cases where ISO 3166-1
        reassigns a deprecated value in the registry.

   17.  The redundant and grandfathered entries together form the
        complete list of tags registered under [RFC3066].  The redundant
        tags are those previously registered tags that can now be formed
        using the subtags defined in the registry.  The grandfathered
        entries include those that can never be legal because they are
        'irregular' (that is, they do not match the 'langtag' production
        in Figure 1), are limited by rule (subtags such as 'nyn' and
        'min' look like the extlang production, but cannot be registered
        as extended language subtags), or their subtags are
        inappropriate for registration.  All of the grandfathered tags
        are listed in either the 'regular' or the 'irregular'
        productions in the ABNF.  Under [RFC4646] it was possible for
        grandfathered tags to become redundant.  However, all of the
        tags for which this was possible became redundant before this
        document was produced.  So the set of redundant and
        grandfathered tags is now permanent and immutable: new entries
        of either type MUST NOT be added and existing entries MUST NOT
        be removed.  The decision-making process about which tags were
        initially grandfathered and which were made redundant is
        described in [RFC4645].

        Many of the grandfathered tags are deprecated -- indeed, they
        were deprecated even before [RFC4646].  For example, the tag
        "art-lojban" was deprecated in favor of the primary language
        subtag 'jbo'.  These tags could have been made 'redundant' by
        registering some of their subtags as 'variants'.  The 'variant-
        like' subtags in the grandfathered registrations SHALL NOT be
        registered in the future, even with a similar or identical
        meaning.

3.5.  Registration Procedure for Subtags

   The procedure given here MUST be used by anyone who wants to use a
   subtag not currently in the IANA Language Subtag Registry or who
   wishes to add, modify, update, or remove information in existing
   records as permitted by this document.

   Only subtags of type 'language' and 'variant' will be considered for
   independent registration of new subtags.  Subtags needed for



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   stability and subtags necessary to keep the registry synchronized
   with ISO 639, ISO 15924, ISO 3166, and UN M.49 within the limits
   defined by this document also use this process, as described in
   Section 3.3 and subject to stability provisions as described in
   Section 3.4.

   Registration requests are accepted relating to information in the
   'Comments', 'Deprecated', 'Description', 'Prefix', 'Preferred-Value',
   'Macrolanguage', or 'Suppress-Script' fields in a subtag's record as
   described in Section 3.4.  Changes to all other fields in the IANA
   registry are NOT permitted.

   Registering a new subtag or requesting modifications to an existing
   tag or subtag starts with the requester filling out the registration
   form reproduced below.  Note that each response is not limited in
   size so that the request can adequately describe the registration.
   The fields in the "Record Requested" section need to follow the
   requirements in Section 3.1 before the record will be approved.

   LANGUAGE SUBTAG REGISTRATION FORM
   1. Name of requester:
   2. E-mail address of requester:
   3. Record Requested:

      Type:
      Subtag:
      Description:
      Prefix:
      Preferred-Value:
      Deprecated:
      Suppress-Script:
      Macrolanguage:
      Comments:

   4. Intended meaning of the subtag:
   5. Reference to published description
      of the language (book or article):
   6. Any other relevant information:

              Figure 5: The Language Subtag Registration Form

   Examples of completed registration forms can be found in Appendix B.
   A complete list of approved registration forms is online through
   http://www.iana.org; readers should note that the Language Tag
   Registry is now obsolete and should instead look for the Language
   Subtag Registry.





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   The subtag registration form MUST be sent to
   <ietf-languages@iana.org>.  Registration requests receive a two-week
   review period before being approved and submitted to IANA for
   inclusion in the registry.  If modifications are made to the request
   during the course of the registration process (such as corrections to
   meet the requirements in Section 3.1 or to make the 'Description'
   fields unique for the given record type), the modified form MUST also
   be sent to <ietf-languages@iana.org> at least one week prior to
   submission to IANA.

   The ietf-languages list is an open list and can be joined by sending
   a request to <ietf-languages-request@iana.org>.  The list can be
   hosted by IANA or any third party at the request of IESG.

   Before forwarding any registration to IANA, the Language Subtag
   Reviewer MUST ensure that all requirements in this document are met.
   This includes ensuring that values in the 'Subtag' field match case
   according to the description in Section 3.1.4 and that 'Description'
   fields are unique for the given record type as described in
   Section 3.1.5.  The Reviewer MUST also ensure that an appropriate
   File-Date record is included in the request, to assist IANA when
   updating the registry (see Section 5.1).

   Some fields in both the registration form as well as the registry
   record itself permit the use of non-ASCII characters.  Registration
   requests SHOULD use the UTF-8 encoding for consistency and clarity.
   However, since some mail clients do not support this encoding, other
   encodings MAY be used for the registration request.  The Language
   Subtag Reviewer is responsible for ensuring that the proper Unicode
   characters appear in both the archived request form and the registry
   record.  In the case of a transcription or encoding error by IANA,
   the Language Subtag Reviewer will request that the registry be
   repaired, providing any necessary information to assist IANA.

   Extended language subtags (type 'extlang'), by definition, are always
   encompassed by another language.  All records of type 'extlang' MUST,
   therefore, contain a 'Prefix' field at the time of registration.
   This 'Prefix' field can never be altered or removed, and requests to
   do so MUST be rejected.

   Variant subtags are usually registered for use with a particular
   range of language tags, and variant subtags based on the terminology
   of the language to which they are apply are encouraged.  For example,
   the subtag 'rozaj' (Resian) is intended for use with language tags
   that start with the primary language subtag "sl" (Slovenian), since
   Resian is a dialect of Slovenian.  Thus, the subtag 'rozaj' would be
   appropriate in tags such as "sl-Latn-rozaj" or "sl-IT-rozaj".  This
   information is stored in the 'Prefix' field in the registry.  Variant



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   registration requests SHOULD include at least one 'Prefix' field in
   the registration form.

   Requests to assign an additional record of a given type with an
   existing subtag value MUST be rejected.  For example, the variant
   subtag 'rozaj' already exists in the registry, so adding a second
   record of type 'variant' with the subtag 'rozaj' is prohibited.

   The 'Prefix' field for a given registered variant subtag exists in
   the IANA registry as a guide to usage.  Additional 'Prefix' fields
   MAY be added by filing an additional registration form.  In that
   form, the "Any other relevant information:" field MUST indicate that
   it is the addition of a prefix.

   Requests to add a 'Prefix' field to a variant subtag that imply a
   different semantic meaning SHOULD be rejected.  For example, a
   request to add the prefix "de" to the subtag '1994' so that the tag
   "de-1994" represented some German dialect or orthographic form would
   be rejected.  The '1994' subtag represents a particular Slovenian
   orthography, and the additional registration would change or blur the
   semantic meaning assigned to the subtag.  A separate subtag SHOULD be
   proposed instead.

   Requests to add a 'Prefix' to a variant subtag that has no current
   'Prefix' field MUST be rejected.  Variants are registered with no
   prefix because they are potentially useful with many or even all
   languages.  Adding one or more 'Prefix' fields would be potentially
   harmful to the use of the variant, since it dramatically reduces the
   scope of the subtag (which is not allowed under the stability rules
   (Section 3.4) as opposed to broadening the scope of the subtag, which
   is what the addition of a 'Prefix' normally does.  An example of such
   a "no-prefix" variant is the subtag 'fonipa', which represents the
   International Phonetic Alphabet, a scheme that can be used to
   transcribe many languages.

   The 'Description' fields provided in the request MUST contain at
   least one description written or transcribed into the Latin script;
   the request MAY also include additional 'Description' fields in any
   script or language.  The 'Description' field is used for
   identification purposes and doesn't necessarily represent the actual
   native name of the language or variation.  It also doesn't have to be
   in any particular language, but SHOULD be both suitable and
   sufficient to identify the item in the record.  The Language Subtag
   Reviewer will check and edit any proposed 'Description' fields so as
   to ensure uniqueness and prevent collisions with 'Description' fields
   in other records of the same type.  If this occurs in an independent
   registration request, the Language Subtag Reviewer MUST resubmit the
   record to <ietf-languages@iana.org>, treating it as a modification of



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   a request due to discussion, as described in Section 3.5, unless the
   request's sole purpose is to introduce a duplicate 'Description'
   field, in which case the request SHALL be rejected.

   The 'Description' field is not guaranteed to be stable.  Corrections
   or clarifications of intent are examples of possible changes.
   Attempts to provide translations or transcriptions of entries in the
   registry (which, by definition, provide no new information) are
   unlikely to be approved.

   Soon after the two-week review period has passed, the Language Subtag
   Reviewer MUST take one of the following actions:

   o  Explicitly accept the request and forward the form containing the
      record to be inserted or modified to <iana@iana.org> according to
      the procedure described in Section 3.3.

   o  Explicitly reject the request because of significant objections
      raised on the list or due to problems with constraints in this
      document (which MUST be explicitly cited).

   o  Extend the review period by granting an additional two-week
      increment to permit further discussion.  After each two-week
      increment, the Language Subtag Reviewer MUST indicate on the list
      whether the registration has been accepted, rejected, or extended.

   Note that the Language Subtag Reviewer MAY raise objections on the
   list if he or she so desires.  The important thing is that the
   objection MUST be made publicly.

   Sometimes the request needs to be modified as a result of discussion
   during the review period or due to requirements in this document.
   The applicant, Language Subtag Reviewer, or others MAY submit a
   modified version of the completed registration form, which will be
   considered in lieu of the original request with the explicit approval
   of the applicant.  Such changes do not restart the two-week
   discussion period, although an application containing the final
   record submitted to IANA MUST appear on the list at least one week
   prior to the Language Subtag Reviewer forwarding the record to IANA.
   The applicant MAY modify a rejected application with more appropriate
   or additional information and submit it again; this starts a new two-
   week comment period.

   Registrations initiated due to the provisions of Section 3.3 or
   Section 3.4 SHALL NOT be rejected altogether (since they have to
   ultimately appear in the registry) and SHOULD be completed as quickly
   as possible.  The review process allows list members to comment on
   the specific information in the form and the record it contains and



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   thus help ensure that it is correct and consistent.  The Language
   Subtag Reviewer MAY reject a specific version of the form, but MUST
   propose a suitable replacement, extending the review period as
   described above, until the form is in a format worthy of the
   reviewer's approval and meets with rough consensus of the list.

   Decisions made by the Language Subtag Reviewer MAY be appealed to the
   IESG [RFC2028] under the same rules as other IETF decisions
   [RFC2026].  This includes a decision to extend the review period or
   the failure to announce a decision in a clear and timely manner.

   The approved records appear in the Language Subtag Registry.  The
   approved registration forms are available online from
   http://www.iana.org.

   Updates or changes to existing records follow the same procedure as
   new registrations.  The Language Subtag Reviewer decides whether
   there is consensus to update the registration following the two-week
   review period; normally, objections by the original registrant will
   carry extra weight in forming such a consensus.

   Registrations are permanent and stable.  Once registered, subtags
   will not be removed from the registry and will remain a valid way in
   which to specify a specific language or variant.

   Note: The purpose of the "Reference to published description" section
   in the registration form is to aid in verifying whether a language is
   registered or to which language or language variation a particular
   subtag refers.  In most cases, reference to an authoritative grammar
   or dictionary of that language will be useful; in cases where no such
   work exists, other well-known works describing that language or in
   that language MAY be appropriate.  The Language Subtag Reviewer
   decides what constitutes "good enough" reference material.  This
   requirement is not intended to exclude particular languages or
   dialects due to the size of the speaker population or lack of a
   standardized orthography.  Minority languages will be considered
   equally on their own merits.

3.6.  Possibilities for Registration

   Possibilities for registration of subtags or information about
   subtags include:

   o  Primary language subtags for languages not listed in ISO 639 that
      are not variants of any listed or registered language MAY be
      registered.  At the time this document was created, there were no
      examples of this form of subtag.  Before attempting to register a
      language subtag, there MUST be an attempt to register the language



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      with ISO 639.  Subtags MUST NOT be registered for languages
      defined by codes that exist in ISO 639-1, ISO 639-2, or ISO 639-3;
      that are under consideration by the ISO 639 registration
      authorities; or that have never been attempted for registration
      with those authorities.  If ISO 639 has previously rejected a
      language for registration, it is reasonable to assume that there
      must be additional, very compelling evidence of need before it
      will be registered as a primary language subtag in the IANA
      registry (to the extent that it is very unlikely that any subtags
      will be registered of this type).

   o  Dialect or other divisions or variations within a language, its
      orthography, writing system, regional or historical usage,
      transliteration or other transformation, or distinguishing
      variation MAY be registered as variant subtags.  An example is the
      'rozaj' subtag (the Resian dialect of Slovenian).

   o  The addition or maintenance of fields (generally of an
      informational nature) in tag or subtag records as described in
      Section 3.1 is allowed.  Such changes are subject to the stability
      provisions in Section 3.4.  This includes 'Description',
      'Comments', 'Deprecated', and 'Preferred-Value' fields for
      obsolete or withdrawn codes, or the addition of 'Suppress-Script'
      or 'Macrolanguage' fields to primary language subtags, as well as
      other changes permitted by this document, such as the addition of
      an appropriate 'Prefix' field to a variant subtag.

   o  The addition of records and related field value changes necessary
      to reflect assignments made by ISO 639, ISO 15924, ISO 3166-1, and
      UN M.49 as described in Section 3.4 is allowed.

   Subtags proposed for registration that would cause all or part of a
   grandfathered tag to become redundant but whose meaning conflicts
   with or alters the meaning of the grandfathered tag MUST be rejected.

   This document leaves the decision on what subtags or changes to
   subtags are appropriate (or not) to the registration process
   described in Section 3.5.

   Note: Four-character primary language subtags are reserved to allow
   for the possibility of alpha4 codes in some future addition to the
   ISO 639 family of standards.

   ISO 639 defines a registration authority for additions to and changes
   in the list of languages in ISO 639.  This agency is:






Phillips & Davis         Best Current Practice                 [Page 47]

RFC 5646                     Language Tags                September 2009


   International Information Centre for Terminology (Infoterm)
   Aichholzgasse 6/12, AT-1120
   Wien, Austria
   Phone: +43 1 26 75 35 Ext. 312 Fax: +43 1 216 32 72

   ISO 639-2 defines a registration authority for additions to and
   changes in the list of languages in ISO 639-2.  This agency is:

   Library of Congress
   Network Development and MARC Standards Office
   Washington, DC 20540, USA
   Phone: +1 202 707 6237 Fax: +1 202 707 0115
   URL: http://www.loc.gov/standards/iso639-2

   ISO 639-3 defines a registration authority for additions to and
   changes in the list of languages in ISO 639-3.  This agency is:

   SIL International
   ISO 639-3 Registrar
   7500 W. Camp Wisdom Rd.
   Dallas, TX 75236, USA
   Phone: +1 972 708 7400, ext. 2293
   Fax: +1 972 708 7546
   Email: iso639-3@sil.org
   URL: http://www.sil.org/iso639-3

   ISO 639-5 defines a registration authority for additions to and
   changes in the list of languages in ISO 639-5.  This agency is the
   same as for ISO 639-2 and is:

   Library of Congress
   Network Development and MARC Standards Office
   Washington, DC 20540, USA
   Phone: +1 202 707 6237
   Fax: +1 202 707 0115
   URL: http://www.loc.gov/standards/iso639-5

   The maintenance agency for ISO 3166-1 (country codes) is:

   ISO 3166 Maintenance Agency
   c/o International Organization for Standardization
   Case postale 56
   CH-1211 Geneva 20, Switzerland
   Phone: +41 22 749 72 33 Fax: +41 22 749 73 49
   URL: http://www.iso.org/iso/en/prods-services/iso3166ma/index.html






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   The registration authority for ISO 15924 (script codes) is:

   Unicode Consortium
   Box 391476
   Mountain View, CA 94039-1476, USA
   URL: http://www.unicode.org/iso15924

   The Statistics Division of the United Nations Secretariat maintains
   the Standard Country or Area Codes for Statistical Use and can be
   reached at:

   Statistical Services Branch
   Statistics Division
   United Nations, Room DC2-1620
   New York, NY 10017, USA
   Fax: +1-212-963-0623
   Email: statistics@un.org
   URL: http://unstats.un.org/unsd/methods/m49/m49alpha.htm

3.7.  Extensions and the Extensions Registry

   Extension subtags are those introduced by single-character subtags
   ("singletons") other than 'x'.  They are reserved for the generation
   of identifiers that contain a language component and are compatible
   with applications that understand language tags.

   The structure and form of extensions are defined by this document so
   that implementations can be created that are forward compatible with
   applications that might be created using singletons in the future.
   In addition, defining a mechanism for maintaining singletons will
   lend stability to this document by reducing the likely need for
   future revisions or updates.

   Single-character subtags are assigned by IANA using the "IETF Review"
   policy defined by [RFC5226].  This policy requires the development of
   an RFC, which SHALL define the name, purpose, processes, and
   procedures for maintaining the subtags.  The maintaining or
   registering authority, including name, contact email, discussion list
   email, and URL location of the registry, MUST be indicated clearly in
   the RFC.  The RFC MUST specify or include each of the following:

   o  The specification MUST reference the specific version or revision
      of this document that governs its creation and MUST reference this
      section of this document.

   o  The specification and all subtags defined by the specification
      MUST follow the ABNF and other rules for the formation of tags and
      subtags as defined in this document.  In particular, it MUST



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      specify that case is not significant and that subtags MUST NOT
      exceed eight characters in length.

   o  The specification MUST specify a canonical representation.

   o  The specification of valid subtags MUST be available over the
      Internet and at no cost.

   o  The specification MUST be in the public domain or available via a
      royalty-free license acceptable to the IETF and specified in the
      RFC.

   o  The specification MUST be versioned, and each version of the
      specification MUST be numbered, dated, and stable.

   o  The specification MUST be stable.  That is, extension subtags,
      once defined by a specification, MUST NOT be retracted or change
      in meaning in any substantial way.

   o  The specification MUST include, in a separate section, the
      registration form reproduced in this section (below) to be used in
      registering the extension upon publication as an RFC.

   o  IANA MUST be informed of changes to the contact information and
      URL for the specification.

   IANA will maintain a registry of allocated single-character
   (singleton) subtags.  This registry MUST use the record-jar format
   described by the ABNF in Section 3.1.1.  Upon publication of an
   extension as an RFC, the maintaining authority defined in the RFC
   MUST forward this registration form to <iesg@ietf.org>, who MUST
   forward the request to <iana@iana.org>.  The maintaining authority of
   the extension MUST maintain the accuracy of the record by sending an
   updated full copy of the record to <iana@iana.org> with the subject
   line "LANGUAGE TAG EXTENSION UPDATE" whenever content changes.  Only
   the 'Comments', 'Contact_Email', 'Mailing_List', and 'URL' fields MAY
   be modified in these updates.

   Failure to maintain this record, maintain the corresponding registry,
   or meet other conditions imposed by this section of this document MAY
   be appealed to the IESG [RFC2028] under the same rules as other IETF
   decisions (see [RFC2026]) and MAY result in the authority to maintain
   the extension being withdrawn or reassigned by the IESG.








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   %%
   Identifier:
   Description:
   Comments:
   Added:
   RFC:
   Authority:
   Contact_Email:
   Mailing_List:
   URL:
   %%

    Figure 6: Format of Records in the Language Tag Extensions Registry

   'Identifier' contains the single-character subtag (singleton)
   assigned to the extension.  The Internet-Draft submitted to define
   the extension SHOULD specify which letter or digit to use, although
   the IESG MAY change the assignment when approving the RFC.

   'Description' contains the name and description of the extension.

   'Comments' is an OPTIONAL field and MAY contain a broader description
   of the extension.

   'Added' contains the date the extension's RFC was published in the
   "full-date" format specified in [RFC3339].  For example: 2004-06-28
   represents June 28, 2004, in the Gregorian calendar.

   'RFC' contains the RFC number assigned to the extension.

   'Authority' contains the name of the maintaining authority for the
   extension.

   'Contact_Email' contains the email address used to contact the
   maintaining authority.

   'Mailing_List' contains the URL or subscription email address of the
   mailing list used by the maintaining authority.

   'URL' contains the URL of the registry for this extension.

   The determination of whether an Internet-Draft meets the above
   conditions and the decision to grant or withhold such authority rests
   solely with the IESG and is subject to the normal review and appeals
   process associated with the RFC process.

   Extension authors are strongly cautioned that many (including most
   well-formed) processors will be unaware of any special relationships



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   or meaning inherent in the order of extension subtags.  Extension
   authors SHOULD avoid subtag relationships or canonicalization
   mechanisms that interfere with matching or with length restrictions
   that sometimes exist in common protocols where the extension is used.
   In particular, applications MAY truncate the subtags in doing
   matching or in fitting into limited lengths, so it is RECOMMENDED
   that the most significant information be in the most significant
   (left-most) subtags and that the specification gracefully handle
   truncated subtags.

   When a language tag is to be used in a specific, known protocol, it
   is RECOMMENDED that the language tag not contain extensions not
   supported by that protocol.  In addition, note that some protocols
   MAY impose upper limits on the length of the strings used to store or
   transport the language tag.

3.8.  Update of the Language Subtag Registry

   After the adoption of this document, the IANA Language Subtag
   Registry needed an update so that it would contain the complete set
   of subtags valid in a language tag.  [RFC5645] describes the process
   used to create this update.

   Registrations that are in process under the rules defined in
   [RFC4646] when this document is adopted MUST be completed under the
   rules contained in this document.

3.9.  Applicability of the Subtag Registry

   The Language Subtag Registry is the source of data elements used to
   construct language tags, following the rules described in this
   document.  Language tags are designed for indicating linguistic
   attributes of various content, including not only text but also most
   media formats, such as video or audio.  They also form the basis for
   language and locale negotiation in various protocols and APIs.

   The registry is therefore applicable to many applications that need
   some form of language identification, with these limitations:

   o  It is not designed to be the sole data source in the creation of a
      language-selection user interface.  For example, the registry does
      not contain translations for subtag descriptions or for tags
      composed from the subtags.  Sources for localized data based on
      the registry are generally available, notably [CLDR].  Nor does
      the registry indicate which subtag combinations are particularly
      useful or relevant.





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   o  It does not provide information indicating relationships between
      different languages, such as might be used in a user interface to
      select language tags hierarchically, regionally, or on some other
      organizational model.

   o  It does not supply information about potential overlap between
      different language tags, as the notion of what constitutes a
      language is not precise: several different language tags might be
      reasonable choices for the same given piece of content.

   o  It does not contain information about appropriate fallback choices
      when performing language negotiation.  A good fallback language
      might be linguistically unrelated to the specified language.  The
      fact that one language is often used as a fallback language for
      another is usually a result of outside factors, such as geography,
      history, or culture -- factors that might not apply in all cases.
      For example, most people who use Breton (a Celtic language used in
      the Northwest of France) would probably prefer to be served French
      (a Romance language) if Breton isn't available.

4.  Formation and Processing of Language Tags

   This section addresses how to use the information in the registry
   with the tag syntax to choose, form, and process language tags.

4.1.  Choice of Language Tag

   The guiding principle in forming language tags is to "tag content
   wisely."  Sometimes there is a choice between several possible tags
   for the same content.  The choice of which tag to use depends on the
   content and application in question, and some amount of judgment
   might be necessary when selecting a tag.

   Interoperability is best served when the same language tag is used
   consistently to represent the same language.  If an application has
   requirements that make the rules here inapplicable, then that
   application risks damaging interoperability.  It is strongly
   RECOMMENDED that users not define their own rules for language tag
   choice.

   Standards, protocols, and applications that reference this document
   normatively but apply different rules to the ones given in this
   section MUST specify how language tag selection varies from the
   guidelines given here.

   To ensure consistent backward compatibility, this document contains
   several provisions to account for potential instability in the
   standards used to define the subtags that make up language tags.



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   These provisions mean that no valid language tag can become invalid,
   nor will a language tag have a narrower scope in the future (it may
   have a broader scope).  The most appropriate language tag for a given
   application or content item might evolve over time, but once applied,
   the tag itself cannot become invalid or have its meaning wholly
   change.

   A subtag SHOULD only be used when it adds useful distinguishing
   information to the tag.  Extraneous subtags interfere with the
   meaning, understanding, and processing of language tags.  In
   particular, users and implementations SHOULD follow the 'Prefix' and
   'Suppress-Script' fields in the registry (defined in Section 3.1):
   these fields provide guidance on when specific additional subtags
   SHOULD be used or avoided in a language tag.

   The choice of subtags used to form a language tag SHOULD follow these
   guidelines:

   1.  Use as precise a tag as possible, but no more specific than is
       justified.  Avoid using subtags that are not important for
       distinguishing content in an application.

       *  For example, 'de' might suffice for tagging an email written
          in German, while "de-CH-1996" is probably unnecessarily
          precise for such a task.

       *  Note that some subtag sequences might not represent the
          language a casual user might expect.  For example, the Swiss
          German (Schweizerdeutsch) language is represented by "gsw-CH"
          and not by "de-CH".  This latter tag represents German ('de')
          as used in Switzerland ('CH'), also known as Swiss High German
          (Schweizer Hochdeutsch).  Both are real languages, and
          distinguishing between them could be important to an
          application.

   2.  The script subtag SHOULD NOT be used to form language tags unless
       the script adds some distinguishing information to the tag.
       Script subtags were first formally defined in [RFC4646].  Their
       use can affect matching and subtag identification for
       implementations of [RFC1766] or [RFC3066] (which are obsoleted by
       this document), as these subtags appear between the primary
       language and region subtags.  Some applications can benefit from
       the use of script subtags in language tags, as long as the use is
       consistent for a given context.  Script subtags are never
       appropriate for unwritten content (such as audio recordings).
       The field 'Suppress-Script' in the primary or extended language
       record in the registry indicates script subtags that do not add
       distinguishing information for most applications; this field



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       defines when users SHOULD NOT include a script subtag with a
       particular primary language subtag.

       For example, if an implementation selects content using Basic
       Filtering [RFC4647] (originally described in Section 14.4 of
       [RFC2616]) and the user requested the language range "en-US",
       content labeled "en-Latn-US" will not match the request and thus
       not be selected.  Therefore, it is important to know when script
       subtags will customarily be used and when they ought not be used.

       For example:

       *  The subtag 'Latn' should not be used with the primary language
          'en' because nearly all English documents are written in the
          Latin script and it adds no distinguishing information.
          However, if a document were written in English mixing Latin
          script with another script such as Braille ('Brai'), then it
          might be appropriate to choose to indicate both scripts to aid
          in content selection, such as the application of a style
          sheet.

       *  When labeling content that is unwritten (such as a recording
          of human speech), the script subtag should not be used, even
          if the language is customarily written in several scripts.
          Thus, the subtitles to a movie might use the tag "uz-Arab"
          (Uzbek, Arabic script), but the audio track for the same
          language would be tagged simply "uz".  (The tag "uz-Zxxx"
          could also be used where content is not written, as the subtag
          'Zxxx' represents the "Code for unwritten documents".)

   3.  If a tag or subtag has a 'Preferred-Value' field in its registry
       entry, then the value of that field SHOULD be used to form the
       language tag in preference to the tag or subtag in which the
       preferred value appears.

       *  For example, use 'jbo' for Lojban in preference to the
          grandfathered tag "art-lojban".

   4.  Use subtags or sequences of subtags for individual languages in
       preference to subtags for language collections.  A "language
       collection" is a group of languages that are descended from a
       common ancestor, are spoken in the same geographical area, or are
       otherwise related.  Certain language collections are assigned
       codes by [ISO639-5] (and some of these [ISO639-5] codes are also
       defined as collections in [ISO639-2]).  These codes are included
       as primary language subtags in the registry.  Subtags for a
       language collection in the registry have a 'Scope' field with a
       value of 'collection'.  A subtag for a language collection is



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       always preferred to less specific alternatives such as 'mul' and
       'und' (see below), and a subtag representing a language
       collection MAY be used when more specific language information is
       not available.  However, most users and implementations do not
       know there is a relationship between the collection and its
       individual languages.  In addition, the relationship between the
       individual languages in the collection is not well defined; in
       particular, the languages are usually not mutually intelligible.
       Since the subtags are different, a request for the collection
       will typically only produce items tagged with the collection's
       subtag, not items tagged with subtags for the individual
       languages contained in the collection.

       *  For example, collections are interpreted inclusively, so the
          subtag 'gem' (Germanic languages) could, but SHOULD NOT, be
          used with content that would be better tagged with "en"
          (English), "de" (German), or "gsw" (Swiss German, Alemannic).
          While 'gem' collects all of these (and other) languages, most
          implementations will not match 'gem' to the individual
          languages; thus, using the subtag will not produce the desired
          result.

   5.  [ISO639-2] has defined several codes included in the subtag
       registry that require additional care when choosing language
       tags.  In most of these cases, where omitting the language tag is
       permitted, such omission is preferable to using these codes.
       Language tags SHOULD NOT incorporate these subtags as a prefix,
       unless the additional information conveys some value to the
       application.

       *  The 'mul' (Multiple) primary language subtag identifies
          content in multiple languages.  This subtag SHOULD NOT be used
          when a list of languages or individual tags for each content
          element can be used instead.  For example, the 'Content-
          Language' header [RFC3282] allows a list of languages to be
          used, not just a single language tag.

       *  The 'und' (Undetermined) primary language subtag identifies
          linguistic content whose language is not determined.  This
          subtag SHOULD NOT be used unless a language tag is required
          and language information is not available or cannot be
          determined.  Omitting the language tag (where permitted) is
          preferred.  The 'und' subtag might be useful for protocols
          that require a language tag to be provided or where a primary
          language subtag is required (such as in "und-Latn").  The
          'und' subtag MAY also be useful when matching language tags in
          certain situations.




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       *  The 'zxx' (Non-Linguistic, Not Applicable) primary language
          subtag identifies content for which a language classification
          is inappropriate or does not apply.  Some examples might
          include instrumental or electronic music; sound recordings
          consisting of nonverbal sounds; audiovisual materials with no
          narration, dialog, printed titles, or subtitles; machine-
          readable data files consisting of machine languages or
          character codes; or programming source code.

       *  The 'mis' (Uncoded) primary language subtag identifies content
          whose language is known but that does not currently have a
          corresponding subtag.  This subtag SHOULD NOT be used.
          Because the addition of other codes in the future can render
          its application invalid, it is inherently unstable and hence
          incompatible with the stability goals of BCP 47.  It is always
          preferable to use other subtags: either 'und' or (with prior
          agreement) private use subtags.

   6.  Use variant subtags sparingly and in the correct order.  Most
       variant subtags have one or more 'Prefix' fields in the registry
       that express the list of subtags with which they are appropriate.
       Variants SHOULD only be used with subtags that appear in one of
       these 'Prefix' fields.  If a variant lists a second variant in
       one of its 'Prefix' fields, the first variant SHOULD appear
       directly after the second variant in any language tag where both
       occur.  General purpose variants (those with no 'Prefix' fields
       at all) SHOULD appear after any other variant subtags.  Order any
       remaining variants by placing the most significant subtag first.
       If none of the subtags is more significant or no relationship can
       be determined, alphabetize the subtags.  Because variants are
       very specialized, using many of them together generally makes the
       tag so narrow as to override the additional precision gained.
       Putting the subtags into another order interferes with
       interoperability, as well as the overall interpretation of the
       tag.

       For example:

       *  The tag "en-scotland-fonipa" (English, Scottish dialect, IPA
          phonetic transcription) is correctly ordered because
          'scotland' has a 'Prefix' of "en", while 'fonipa' has no
          'Prefix' field.

       *  The tag "sl-IT-rozaj-biske-1994" is correctly ordered: 'rozaj'
          lists "sl" as its sole 'Prefix'; 'biske' lists "sl-rozaj" as
          its sole 'Prefix'.  The subtag '1994' has several prefixes,





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          including "sl-rozaj".  However, it follows both 'rozaj' and
          'biske' because one of its 'Prefix' fields is "sl-rozaj-
          biske".

   7.  The grandfathered tag "i-default" (Default Language) was
       originally registered according to [RFC1766] to meet the needs of
       [RFC2277].  It is not used to indicate a specific language, but
       rather to identify the condition or content used where the
       language preferences of the user cannot be established.  It
       SHOULD NOT be used except as a means of labeling the default
       content for applications or protocols that require default
       language content to be labeled with that specific tag.  It MAY
       also be used by an application or protocol to identify when the
       default language content is being returned.

4.1.1.  Tagging Encompassed Languages

   Some primary language records in the registry have a 'Macrolanguage'
   field (Section 3.1.10) that contains a mapping from each "encompassed
   language" to its macrolanguage.  The 'Macrolanguage' mapping doesn't
   define what the relationship between the encompassed language and its
   macrolanguage is, nor does it define how languages encompassed by the
   same macrolanguage are related to each other.  Two different
   languages encompassed by the same macrolanguage may differ from one
   another more than, say, French and Spanish do.

   A few specific macrolanguages, such as Chinese ('zh') and Arabic
   ('ar'), are handled differently.  See Section 4.1.2.

   The more specific encompassed language subtag SHOULD be used to form
   the language tag, although either the macrolanguage's primary
   language subtag or the encompassed language's subtag MAY be used.
   This means, for example, tagging Plains Cree with 'crk' rather than
   'cr' (Cree), and so forth.

   Each macrolanguage subtag's scope, by definition, includes all of its
   encompassed languages.  Since the relationship between encompassed
   languages varies, users cannot assume that the macrolanguage subtag
   means any particular encompassed language, nor that any given pair of
   encompassed languages are mutually intelligible or otherwise
   interchangeable.

   Applications MAY use macrolanguage information to improve matching or
   language negotiation.  For example, the information that 'sr'
   (Serbian) and 'hr' (Croatian) share a macrolanguage expresses a
   closer relation between those languages than between, say, 'sr'
   (Serbian) and 'ma' (Macedonian).  However, this relationship is not
   guaranteed nor is it exclusive.  For example, Romanian ('ro') and



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   Moldavian ('mo') do not share a macrolanguage, but are far more
   closely related to each other than Cantonese ('yue') and Wu ('wuu'),
   which do share a macrolanguage.

4.1.2.  Using Extended Language Subtags

   To accommodate language tag forms used prior to the adoption of this
   document, language tags provide a special compatibility mechanism:
   the extended language subtag.  Selected languages have been provided
   with both primary and extended language subtags.  These include
   macrolanguages, such as Malay ('ms') and Uzbek ('uz'), that have a
   specific dominant variety that is generally synonymous with the
   macrolanguage.  Other languages, such as the Chinese ('zh') and
   Arabic ('ar') macrolanguages and the various sign languages ('sgn'),
   have traditionally used their primary language subtag, possibly
   coupled with various region subtags or as part of a registered
   grandfathered tag, to indicate the language.

   With the adoption of this document, specific ISO 639-3 subtags became
   available to identify the languages contained within these diverse
   language families or groupings.  This presents a choice of language
   tags where previously none existed:

   o  Each encompassed language's subtag SHOULD be used as the primary
      language subtag.  For example, a document in Mandarin Chinese
      would be tagged "cmn" (the subtag for Mandarin Chinese) in
      preference to "zh" (Chinese).

   o  If compatibility is desired or needed, the encompassed subtag MAY
      be used as an extended language subtag.  For example, a document
      in Mandarin Chinese could be tagged "zh-cmn" instead of either
      "cmn" or "zh".

   o  The macrolanguage or prefixing subtag MAY still be used to form
      the tag instead of the more specific encompassed language subtag.
      That is, tags such as "zh-HK" or "sgn-RU" are still valid.

   Chinese ('zh') provides a useful illustration of this.  In the past,
   various content has used tags beginning with the 'zh' subtag, with
   application-specific meaning being associated with region codes,
   private use sequences, or grandfathered registered values.  This is
   because historically only the macrolanguage subtag 'zh' was available
   for forming language tags.  However, the languages encompassed by the
   Chinese subtag 'zh' are, in the main, not mutually intelligible when
   spoken, and the written forms of these languages also show wide
   variation in form and usage.





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   To provide compatibility, Chinese languages encompassed by the 'zh'
   subtag are in the registry both as primary language subtags and as
   extended language subtags.  For example, the ISO 639-3 code for
   Cantonese is 'yue'.  Content in Cantonese might historically have
   used a tag such as "zh-HK" (since Cantonese is commonly spoken in
   Hong Kong), although that tag actually means any type of Chinese as
   used in Hong Kong.  With the availability of ISO 639-3 codes in the
   registry, content in Cantonese can be directly tagged using the 'yue'
   subtag.  The content can use it as a primary language subtag, as in
   the tag "yue-HK" (Cantonese, Hong Kong).  Or it can use an extended
   language subtag with 'zh', as in the tag "zh-yue-Hant" (Chinese,
   Cantonese, Traditional script).

   As noted above, applications can choose to use the macrolanguage
   subtag to form the tag instead of using the more specific encompassed
   language subtag.  For example, an application with large quantities
   of data already using tags with the 'zh' (Chinese) subtag might
   continue to use this more general subtag even for new data, even
   though the content could be more precisely tagged with 'cmn'
   (Mandarin), 'yue' (Cantonese), 'wuu' (Wu), and so on.  Similarly, an
   application already using tags that start with the 'ar' (Arabic)
   subtag might continue to use this more general subtag even for new
   data, which could be more precisely tagged with 'arb' (Standard
   Arabic).

   In some cases, the encompassed languages had tags registered for them
   during the RFC 3066 era.  Those grandfathered tags not already
   deprecated or rendered redundant were deprecated in the registry upon
   adoption of this document.  As grandfathered values, they remain
   valid for use, and some content or applications might use them.  As
   with other grandfathered tags, since implementations might not be
   able to associate the grandfathered tags with the encompassed
   language subtag equivalents that are recommended by this document,
   implementations are encouraged to canonicalize tags for comparison
   purposes.  Some examples of this include the tags "zh-hakka" (Hakka)
   and "zh-guoyu" (Mandarin or Standard Chinese).

   Sign languages share a mode of communication rather than a linguistic
   heritage.  There are many sign languages that have developed
   independently, and the subtag 'sgn' indicates only the presence of a
   sign language.  A number of sign languages also had grandfathered
   tags registered for them during the RFC 3066 era.  For example, the
   grandfathered tag "sgn-US" was registered to represent 'American Sign
   Language' specifically, without reference to the United States.  This
   is still valid, but deprecated: a document in American Sign Language
   can be labeled either "ase" or "sgn-ase" (the 'ase' subtag is for the
   language called 'American Sign Language').




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4.2.  Meaning of the Language Tag

   The meaning of a language tag is related to the meaning of the
   subtags that it contains.  Each subtag, in turn, implies a certain
   range of expectations one might have for related content, although it
   is not a guarantee.  For example, the use of a script subtag such as
   'Arab' (Arabic script) does not mean that the content contains only
   Arabic characters.  It does mean that the language involved is
   predominantly in the Arabic script.  Thus, a language tag and its
   subtags can encompass a very wide range of variation and yet remain
   appropriate in each particular instance.

   Validity of a tag is not the only factor determining its usefulness.
   While every valid tag has a meaning, it might not represent any real-
   world language usage.  This is unavoidable in a system in which
   subtags can be combined freely.  For example, tags such as
   "ar-Cyrl-CO" (Arabic, Cyrillic script, as used in Colombia) or "tlh-
   Kore-AQ-fonipa" (Klingon, Korean script, as used in Antarctica, IPA
   phonetic transcription) are both valid and unlikely to represent a
   useful combination of language attributes.

   The meaning of a given tag doesn't depend on the context in which it
   appears.  The relationship between a tag's meaning and the
   information objects to which that tag is applied, however, can vary.

   o  For a single information object, the associated language tags
      might be interpreted as the set of languages that is necessary for
      a complete comprehension of the complete object.  Example: Plain
      text documents.

   o  For an aggregation of information objects, the associated language
      tags could be taken as the set of languages used inside components
      of that aggregation.  Examples: Document stores and libraries.

   o  For information objects whose purpose is to provide alternatives,
      the associated language tags could be regarded as a hint that the
      content is provided in several languages and that one has to
      inspect each of the alternatives in order to find its language or
      languages.  In this case, the presence of multiple tags might not
      mean that one needs to be multilingual to get complete
      understanding of the document.  Example: MIME multipart/
      alternative [RFC2046].

   o  For markup languages, such as HTML and XML, language information
      can be added to each part of the document identified by the markup
      structure (including the whole document itself).  For example, one
      could write <span lang="fr">C'est la vie.</span> inside a German
      document; the German-speaking user could then access a French-



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      German dictionary to find out what the marked section meant.  If
      the user were listening to that document through a speech
      synthesis interface, this formation could be used to signal the
      synthesizer to appropriately apply French text-to-speech
      pronunciation rules to that span of text, instead of applying the
      inappropriate German rules.

   o  For markup languages and document formats that allow the audience
      to be identified, a language tag could indicate the audience(s)
      appropriate for that document.  For example, the same HTML
      document described in the preceding bullet might have an HTTP
      header "Content-Language: de" to indicate that the intended
      audience for the file is German (even though three words appear
      and are identified as being in French within it).

   o  For systems and APIs, language tags form the basis for most
      implementations of locale identifiers.  For example, see Unicode's
      CLDR (Common Locale Data Repository) (see UTS #35 [UTS35])
      project.

   Language tags are related when they contain a similar sequence of
   subtags.  For example, if a language tag B contains language tag A as
   a prefix, then B is typically "narrower" or "more specific" than A.
   Thus, "zh-Hant-TW" is more specific than "zh-Hant".

   This relationship is not guaranteed in all cases: specifically,
   languages that begin with the same sequence of subtags are NOT
   guaranteed to be mutually intelligible, although they might be.  For
   example, the tag "az" shares a prefix with both "az-Latn"
   (Azerbaijani written using the Latin script) and "az-Cyrl"
   (Azerbaijani written using the Cyrillic script).  A person fluent in
   one script might not be able to read the other, even though the
   linguistic content (e.g., what would be heard if both texts were read
   aloud) might be identical.  Content tagged as "az" most probably is
   written in just one script and thus might not be intelligible to a
   reader familiar with the other script.

   Similarly, not all subtags specify an actual distinction in language.
   For example, the tags "en-US" and "en-CA" mean, roughly, English with
   features generally thought to be characteristic of the United States
   and Canada, respectively.  They do not imply that a significant
   dialectical boundary exists between any arbitrarily selected point in
   the United States and any arbitrarily selected point in Canada.
   Neither does a particular region subtag imply that linguistic
   distinctions do not exist within that region.






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4.3.  Lists of Languages

   In some applications, a single content item might best be associated
   with more than one language tag.  Examples of such a usage include:

   o  Content items that contain multiple, distinct varieties.  Often
      this is used to indicate an appropriate audience for a given
      content item when multiple choices might be appropriate.  Examples
      of this could include:

      *  Metadata about the appropriate audience for a movie title.  For
         example, a DVD might label its individual audio tracks 'de'
         (German), 'fr' (French), and 'es' (Spanish), but the overall
         title would list "de, fr, es" as its overall audience.

      *  A French/English, English/French dictionary tagged as both "en"
         and "fr" to specify that it applies equally to French and
         English.

      *  A side-by-side or interlinear translation of a document, as is
         commonly done with classical works in Latin or Greek.

   o  Content items that contain a single language but that require
      multiple levels of specificity.  For example, a library might wish
      to classify a particular work as both Norwegian ('no') and as
      Nynorsk ('nn') for audiences capable of appreciating the
      distinction or needing to select content more narrowly.

4.4.  Length Considerations

   There is no defined upper limit on the size of language tags.  While
   historically most language tags have consisted of language and region
   subtags with a combined total length of up to six characters, larger
   tags have always been both possible and have actually appeared in
   use.

   Neither the language tag syntax nor other requirements in this
   document impose a fixed upper limit on the number of subtags in a
   language tag (and thus an upper bound on the size of a tag).  The
   language tag syntax suggests that, depending on the specific
   language, more subtags (and thus a longer tag) are sometimes
   necessary to completely identify the language for certain
   applications; thus, it is possible to envision long or complex subtag
   sequences.







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4.4.1.  Working with Limited Buffer Sizes

   Some applications and protocols are forced to allocate fixed buffer
   sizes or otherwise limit the length of a language tag.  A conformant
   implementation or specification MAY refuse to support the storage of
   language tags that exceed a specified length.  Any such limitation
   SHOULD be clearly documented, and such documentation SHOULD include
   what happens to longer tags (for example, whether an error value is
   generated or the language tag is truncated).  A protocol that allows
   tags to be truncated at an arbitrary limit, without giving any
   indication of what that limit is, has the potential to cause harm by
   changing the meaning of tags in substantial ways.

   In practice, most language tags do not require more than a few
   subtags and will not approach reasonably sized buffer limitations;
   see Section 4.1.

   Some specifications or protocols have limits on tag length but do not
   have a fixed length limitation.  For example, [RFC2231] has no
   explicit length limitation: the length available for the language tag
   is constrained by the length of other header components (such as the
   charset's name) coupled with the 76-character limit in [RFC2047].
   Thus, the "limit" might be 50 or more characters, but it could
   potentially be quite small.

   The considerations for assigning a buffer limit are:

      Implementations SHOULD NOT truncate language tags unless the
      meaning of the tag is purposefully being changed, or unless the
      tag does not fit into a limited buffer size specified by a
      protocol for storage or transmission.

      Implementations SHOULD warn the user when a tag is truncated since
      truncation changes the semantic meaning of the tag.

      Implementations of protocols or specifications that are space
      constrained but do not have a fixed limit SHOULD use the longest
      possible tag in preference to truncation.

      Protocols or specifications that specify limited buffer sizes for
      language tags MUST allow for language tags of at least 35
      characters.  Note that [RFC4646] recommended a minimum field size
      of 42 characters because it included all three elements of the
      'extlang' production.  Two of these are now permanently reserved,
      so a registered primary language subtag of the maximum length of 8
      characters is now longer than the longest language-extlang
      combination.  Protocols or specifications that commonly use




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      extensions or private use subtags might wish to reserve or
      recommend a longer "minimum buffer" size.

   The following illustration shows how the 35-character recommendation
   was derived:

   language      =  8 ; longest allowed registered value
                      ;   longer than primary+extlang
                      ;   which requires 7 characters
   script        =  5 ; if not suppressed: see Section 4.1
   region        =  4 ; UN M.49 numeric region code
                      ;   ISO 3166-1 codes require 3
   variant1      =  9 ; needs 'language' as a prefix
   variant2      =  9 ; very rare, as it needs
                      ;   'language-variant1' as a prefix

   total         = 35 characters

              Figure 7: Derivation of the Limit on Tag Length

4.4.2.  Truncation of Language Tags

   Truncation of a language tag alters the meaning of the tag, and thus
   SHOULD be avoided.  However, truncation of language tags is sometimes
   necessary due to limited buffer sizes.  Such truncation MUST NOT
   permit a subtag to be chopped off in the middle or the formation of
   invalid tags (for example, one ending with the "-" character).

   This means that applications or protocols that truncate tags MUST do
   so by progressively removing subtags along with their preceding "-"
   from the right side of the language tag until the tag is short enough
   for the given buffer.  If the resulting tag ends with a single-
   character subtag, that subtag and its preceding "-" MUST also be
   removed.  For example:

   Tag to truncate: zh-Latn-CN-variant1-a-extend1-x-wadegile-private1
   1. zh-Latn-CN-variant1-a-extend1-x-wadegile
   2. zh-Latn-CN-variant1-a-extend1
   3. zh-Latn-CN-variant1
   4. zh-Latn-CN
   5. zh-Latn
   6. zh

                    Figure 8: Example of Tag Truncation







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4.5.  Canonicalization of Language Tags

   Since a particular language tag can be used by many processes,
   language tags SHOULD always be created or generated in canonical
   form.

   A language tag is in 'canonical form' when the tag is well-formed
   according to the rules in Sections 2.1 and 2.2 and it has been
   canonicalized by applying each of the following steps in order, using
   data from the IANA registry (see Section 3.1):

   1.  Extension sequences are ordered into case-insensitive ASCII order
       by singleton subtag.

       *  For example, the subtag sequence '-a-babble' comes before
          '-b-warble'.

   2.  Redundant or grandfathered tags are replaced by their 'Preferred-
       Value', if there is one.

       *  The field-body of the 'Preferred-Value' for grandfathered and
          redundant tags is an "extended language range" [RFC4647] and
          might consist of more than one subtag.

       *  'Preferred-Value' fields in the registry provide mappings from
          deprecated tags to modern equivalents.  Many of these were
          created before the adoption of this document (such as the
          mapping of "no-nyn" to "nn" or "i-klingon" to "tlh").  Others
          are the result of later registrations or additions to the
          registry as permitted or required by this document (for
          example, "zh-hakka" was deprecated in favor of the ISO 639-3
          code 'hak' when this document was adopted).

   3.  Subtags are replaced by their 'Preferred-Value', if there is one.
       For extlangs, the original primary language subtag is also
       replaced if there is a primary language subtag in the 'Preferred-
       Value'.

       *  The field-body of the 'Preferred-Value' for extlangs is an
          "extended language range" and typically maps to a primary
          language subtag.  For example, the subtag sequence "zh-hak"
          (Chinese, Hakka) is replaced with the subtag 'hak' (Hakka).

       *  Most of the non-extlang subtags are either Region subtags
          where the country name or designation has changed or clerical
          corrections to ISO 639-1.





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   The canonical form contains no 'extlang' subtags.  There is an
   alternate 'extlang form' that maintains or reinstates extlang
   subtags.  This form can be useful in environments where the presence
   of the 'Prefix' subtag is considered beneficial in matching or
   selection (see Section 4.1.2).

   A language tag is in 'extlang form' when the tag is well-formed
   according to the rules in Sections 2.1 and 2.2 and it has been
   processed by applying each of the following two steps in order, using
   data from the IANA registry:

   1.  The language tag is first transformed into canonical form, as
       described above.

   2.  If the language tag starts with a primary language subtag that is
       also an extlang subtag, then the language tag is prepended with
       the extlang's 'Prefix'.

       *  For example, "hak-CN" (Hakka, China) has the primary language
          subtag 'hak', which in turn has an 'extlang' record with a
          'Prefix' 'zh' (Chinese).  The extlang form is "zh-hak-CN"
          (Chinese, Hakka, China).

       *  Note that Step 2 (prepending a prefix) can restore a subtag
          that was removed by Step 1 (canonicalizing).

   Example: The language tag "en-a-aaa-b-ccc-bbb-x-xyz" is in canonical
   form, while "en-b-ccc-bbb-a-aaa-X-xyz" is well-formed and potentially
   valid (extensions 'a' and 'b' are not defined as of the publication
   of this document) but not in canonical form (the extensions are not
   in alphabetical order).

   Example: Although the tag "en-BU" (English as used in Burma)
   maintains its validity, the language tag "en-BU" is not in canonical
   form because the 'BU' subtag has a canonical mapping to 'MM'
   (Myanmar).

   Canonicalization of language tags does not imply anything about the
   use of upper- or lowercase letters when processing or comparing
   subtags (and as described in Section 2.1).  All comparisons MUST be
   performed in a case-insensitive manner.

   When performing canonicalization of language tags, processors MAY
   regularize the case of the subtags (that is, this process is
   OPTIONAL), following the case used in the registry (see
   Section 2.1.1).





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   If more than one variant appears within a tag, processors MAY reorder
   the variants to obtain better matching behavior or more consistent
   presentation.  Reordering of the variants SHOULD follow the
   recommendations for variant ordering in Section 4.1.

   If the field 'Deprecated' appears in a registry record without an
   accompanying 'Preferred-Value' field, then that tag or subtag is
   deprecated without a replacement.  These values are canonical when
   they appear in a language tag.  However, tags that include these
   values SHOULD NOT be selected by users or generated by
   implementations.

   An extension MUST define any relationships that exist between the
   various subtags in the extension and thus MAY define an alternate
   canonicalization scheme for the extension's subtags.  Extensions MAY
   define how the order of the extension's subtags is interpreted.  For
   example, an extension could define that its subtags are in canonical
   order when the subtags are placed into ASCII order: that is, "en-a-
   aaa-bbb-ccc" instead of "en-a-ccc-bbb-aaa".  Another extension might
   define that the order of the subtags influences their semantic
   meaning (so that "en-b-ccc-bbb-aaa" has a different value from "en-b-
   aaa-bbb-ccc").  However, extension specifications SHOULD be designed
   so that they are tolerant of the typical processes described in
   Section 3.7.

4.6.  Considerations for Private Use Subtags

   Private use subtags, like all other subtags, MUST conform to the
   format and content constraints in the ABNF.  Private use subtags have
   no meaning outside the private agreement between the parties that
   intend to use or exchange language tags that employ them.  The same
   subtags MAY be used with a different meaning under a separate private
   agreement.  They SHOULD NOT be used where alternatives exist and
   SHOULD NOT be used in content or protocols intended for general use.

   Private use subtags are simply useless for information exchange
   without prior arrangement.  The value and semantic meaning of private
   use tags and of the subtags used within such a language tag are not
   defined by this document.

   Private use sequences introduced by the 'x' singleton are completely
   opaque to users or implementations outside of the private use
   agreement.  So, in addition to private use subtag sequences
   introduced by the singleton subtag 'x', the Language Subtag Registry
   provides private use language, script, and region subtags derived
   from the private use codes assigned by the underlying standards.
   These subtags are valid for use in forming language tags; they are
   RECOMMENDED over the 'x' singleton private use subtag sequences



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   because they convey more information via their linkage to the
   language tag's inherent structure.

   For example, the region subtags 'AA', 'ZZ', and those in the ranges
   'QM'-'QZ' and 'XA'-'XZ' (derived from the ISO 3166-1 private use
   codes) can be used to form a language tag.  A tag such as
   "zh-Hans-XQ" conveys a great deal of public, interchangeable
   information about the language material (that it is Chinese in the
   simplified Chinese script and is suitable for some geographic region
   'XQ').  While the precise geographic region is not known outside of
   private agreement, the tag conveys far more information than an
   opaque tag such as "x-somelang" or even "zh-Hans-x-xq" (where the
   'xq' subtag's meaning is entirely opaque).

   However, in some cases content tagged with private use subtags can
   interact with other systems in a different and possibly unsuitable
   manner compared to tags that use opaque, privately defined subtags,
   so the choice of the best approach sometimes depends on the
   particular domain in question.

5.  IANA Considerations

   This section deals with the processes and requirements necessary for
   IANA to maintain the subtag and extension registries as defined by
   this document and in accordance with the requirements of [RFC5226].

   The impact on the IANA maintainers of the two registries defined by
   this document will be a small increase in the frequency of new
   entries or updates.  IANA also is required to create a new mailing
   list (described below in Section 5.1) to announce registry changes
   and updates.

5.1.  Language Subtag Registry

   IANA updated the registry using instructions and content provided in
   a companion document [RFC5645].  The criteria and process for
   selecting the updated set of records are described in that document.
   The updated set of records represents no impact on IANA, since the
   work to create it will be performed externally.

   Future work on the Language Subtag Registry includes the following
   activities:

   o  Inserting or replacing whole records.  These records are
      preformatted for IANA by the Language Subtag Reviewer, as
      described in Section 3.3.

   o  Archiving and making publicly available the registration forms.



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   o  Announcing each updated version of the registry on the
      "ietf-languages-announcements@iana.org" mailing list.

   Each registration form sent to IANA contains a single record for
   incorporation into the registry.  The form will be sent to
   <iana@iana.org> by the Language Subtag Reviewer.  It will have a
   subject line indicating whether the enclosed form represents an
   insertion of a new record (indicated by the word "INSERT" in the
   subject line) or a replacement of an existing record (indicated by
   the word "MODIFY" in the subject line).  At no time can a record be
   deleted from the registry.

   IANA will extract the record from the form and place the inserted or
   modified record into the appropriate section of the Language Subtag
   Registry, grouping the records by their 'Type' field.  Inserted
   records can be placed anywhere within the appropriate section; there
   is no guarantee that the registry's records will be placed in any
   particular order except that they will always be grouped by 'Type'.
   Modified records overwrite the record they replace.

   Whenever an entry is created or modified in the registry, the 'File-
   Date' record at the start of the registry is updated to reflect the
   most recent modification date.  The date format SHALL be the "full-
   date" format of [RFC3339].  The date SHALL be the date on which that
   version of the registry was first published by IANA.  There SHALL be
   at most one version of the registry published in a day.  A 'File-
   Date' record is also included in each request to IANA to insert or
   modify records, indicating the acceptance date of the records in the
   request.

   The updated registry file MUST use the UTF-8 character encoding, and
   IANA MUST check the registry file for proper encoding.  Non-ASCII
   characters can be sent to IANA by attaching the registration form to
   the email message or by using various encodings in the mail message
   body (UTF-8 is recommended).  IANA will verify any unclear or
   corrupted characters with the Language Subtag Reviewer prior to
   posting the updated registry.

   IANA will also archive and make publicly available from
   http://www.iana.org each registration form.  Note that multiple
   registrations can pertain to the same record in the registry.

   Developers who are dependent upon the Language Subtag Registry
   sometimes would like to be informed of changes in the registry so
   that they can update their implementations.  When any change is made
   to the Language Subtag Registry, IANA will send an announcement
   message to <ietf-languages-announcements@iana.org> (a self-
   subscribing list to which only IANA can post).



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5.2.  Extensions Registry

   The Language Tag Extensions Registry can contain at most 35 records,
   and thus changes to this registry are expected to be very infrequent.

   Future work by IANA on the Language Tag Extensions Registry is
   limited to two cases.  First, the IESG MAY request that new records
   be inserted into this registry from time to time.  These requests
   MUST include the record to insert in the exact format described in
   Section 3.7.  In addition, there MAY be occasional requests from the
   maintaining authority for a specific extension to update the contact
   information or URLs in the record.  These requests MUST include the
   complete, updated record.  IANA is not responsible for validating the
   information provided, only that it is properly formatted.  IANA
   SHOULD take reasonable steps to ascertain that the request comes from
   the maintaining authority named in the record present in the
   registry.

6.  Security Considerations

   Language tags used in content negotiation, like any other information
   exchanged on the Internet, might be a source of concern because they
   might be used to infer the nationality of the sender, and thus
   identify potential targets for surveillance.

   This is a special case of the general problem that anything sent is
   visible to the receiving party and possibly to third parties as well.
   It is useful to be aware that such concerns can exist in some cases.

   The evaluation of the exact magnitude of the threat, and any possible
   countermeasures, is left to each application protocol (see BCP 72
   [RFC3552] for best current practice guidance on security threats and
   defenses).

   The language tag associated with a particular information item is of
   no consequence whatsoever in determining whether that content might
   contain possible homographs.  The fact that a text is tagged as being
   in one language or using a particular script subtag provides no
   assurance whatsoever that it does not contain characters from scripts
   other than the one(s) associated with or specified by that language
   tag.

   Since there is no limit to the number of variant, private use, and
   extension subtags, and consequently no limit on the possible length
   of a tag, implementations need to guard against buffer overflow
   attacks.  See Section 4.4 for details on language tag truncation,
   which can occur as a consequence of defenses against buffer overflow.




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   To prevent denial-of-service attacks, applications SHOULD NOT depend
   on either the Language Subtag Registry or the Language Tag Extensions
   Registry being always accessible.  Additionally, although the
   specification of valid subtags for an extension (see Section 3.7)
   MUST be available over the Internet, implementations SHOULD NOT
   mechanically depend on those sources being always accessible.

   The registries specified in this document are not suitable for
   frequent or real-time access to, or retrieval of, the full registry
   contents.  Most applications do not need registry data at all.  For
   others, being able to validate or canonicalize language tags as of a
   particular registry date will be sufficient, as the registry contents
   change only occasionally.  Changes are announced to
   <ietf-languages-announcements@iana.org>.  This mailing list is
   intended for interested organizations and individuals, not for bulk
   subscription to trigger automatic software updates.  The size of the
   registry makes it unsuitable for automatic software updates.
   Implementers considering integrating the Language Subtag Registry in
   an automatic updating scheme are strongly advised to distribute only
   suitably encoded differences, and only via their own infrastructure
   -- not directly from IANA.

   Changes, or the absence thereof, can also easily be detected by
   looking at the 'File-Date' record at the start of the registry, or by
   using features of the protocol used for downloading, without having
   to download the full registry.  At the time of publication of this
   document, IANA is making the Language Tag Registry available over
   HTTP 1.1.  The proper way to update a local copy of the Language
   Subtag Registry using HTTP 1.1 is to use a conditional GET [RFC2616].

7.  Character Set Considerations

   The syntax in this document requires that language tags use only the
   characters A-Z, a-z, 0-9, and HYPHEN-MINUS, which are present in most
   character sets, so the composition of language tags shouldn't have
   any character set issues.

   The rendering of text based on the language tag is not addressed
   here.  Historically, some processes have relied on the use of
   character set/encoding information (or other external information) in
   order to infer how a specific string of characters should be
   rendered.  Notably, this applies to language- and culture-specific
   variations of Han ideographs as used in Japanese, Chinese, and
   Korean, where use of, for example, a Japanese character encoding such
   as EUC-JP implies that the text itself is in Japanese.  When language
   tags are applied to spans of text, rendering engines might be able to
   use that information to better select fonts or make other rendering




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   choices, particularly where languages with distinct writing
   traditions use the same characters.

8.  Changes from RFC 4646

   The main goal for this revision of RFC 4646 was to incorporate two
   new parts of ISO 639 (ISO 639-3 and ISO 639-5) and their attendant
   sets of language codes into the IANA Language Subtag Registry.  This
   permits the identification of many more languages and language
   collections than previously supported.

   The specific changes in this document to meet these goals are:

   o  Defined the incorporation of ISO 639-3 and ISO 639-5 codes for use
      as primary and extended language subtags.  It also permanently
      reserves and disallows the use of additional 'extlang' subtags.
      The changes necessary to achieve this were:

      *  Modified the ABNF comments.

      *  Updated various registration and stability requirements
         sections to reference ISO 639-3 and ISO 639-5 in addition to
         ISO 639-1 and ISO 639-2.

      *  Edited the text to eliminate references to extended language
         subtags where they are no longer used.

      *  Explained the change in the section on extended language
         subtags.

   o  Changed the ABNF related to grandfathered tags.  The irregular
      tags are now listed.  Well-formed grandfathered tags are now
      described by the 'langtag' production, and the 'grandfathered'
      production was removed as a result.  Also: added description of
      both types of grandfathered tags to Section 2.2.8.

   o  Added the paragraph on "collections" to Section 4.1.

   o  Changed the capitalization rules for 'Tag' fields in Section 3.1.

   o  Split Section 3.1 up into subsections.

   o  Modified Section 3.5 to allow 'Suppress-Script' fields to be
      added, modified, or removed via the registration process.  This
      was an erratum from RFC 4646.

   o  Modified examples that used region code 'CS' (formerly Serbia and
      Montenegro) to use 'RS' (Serbia) instead.



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   o  Modified the rules for creating and maintaining record
      'Description' fields to prevent duplicates, including inverted
      duplicates.

   o  Removed the lengthy description of why RFC 4646 was created from
      this section, which also caused the removal of the reference to
      XML Schema.

   o  Modified the text in Section 2.1 to place more emphasis on the
      fact that language tags are not case sensitive.

   o  Replaced the example "fr-Latn-CA" in Section 2.1 with "sr-Latn-RS"
      and "az-Arab-IR" because "fr-Latn-CA" doesn't respect the
      'Suppress-Script' on 'Latn' with 'fr'.

   o  Changed the requirements for well-formedness to make singleton
      repetition checking optional (it is required for validity
      checking) in Section 2.2.9.

   o  Changed the text in Section 2.2.9 referring to grandfathered
      checking to note that the list is now included in the ABNF.

   o  Modified and added text to Section 3.2.  The job description was
      placed first.  A note was added making clear that the Language
      Subtag Reviewer may delegate various non-critical duties,
      including list moderation.  Finally, additional text was added to
      make the appointment process clear and to clarify that decisions
      and performance of the reviewer are appealable.

   o  Added text to Section 3.5 clarifying that the
      ietf-languages@iana.org list is operated by whomever the IESG
      appoints.

   o  Added text to Section 3.1.5 clarifying that the first Description
      in a 'language' record matches the corresponding Reference Name
      for the language in ISO 639-3.

   o  Modified Section 2.2.9 to define classes of conformance related to
      specific tags (formerly 'well-formed' and 'valid' referred to
      implementations).  Notes were added about the removal of 'extlang'
      from the ABNF provided in RFC 4646, allowing for well-formedness
      using this older definition.  Reference to RFC 3066 well-
      formedness was also added.

   o  Added text to the end of Section 3.1.2 noting that future versions
      of this document might add new field types to the registry format
      and recommending that implementations ignore any unrecognized
      fields.



Phillips & Davis         Best Current Practice                 [Page 74]

RFC 5646                     Language Tags                September 2009


   o  Added text about what the lack of a 'Suppress-Script' field means
      in a record to Section 3.1.9.

   o  Added text allowing the correction of misspellings and typographic
      errors to Section 3.1.5.

   o  Added text to Section 3.1.8 disallowing 'Prefix' field conflicts
      (such as circular prefix references).

   o  Modified text in Section 3.5 to require the subtag reviewer to
      announce his/her decision (or extension) following the two-week
      period.  Also clarified that any decision or failure to decide can
      be appealed.

   o  Modified text in Section 4.1 to include the (heretofore anecdotal)
      guiding principle of tag choice, and clarifying the non-use of
      script subtags in non-written applications.

   o  Prohibited multiple use of the same variant in a tag (i.e., "de-
      1901-1901").  Previously, this was only a recommendation
      ("SHOULD").

   o  Removed inappropriate [RFC2119] language from the illustration in
      Section 4.4.1.

   o  Replaced the example of deprecating "zh-guoyu" with "zh-
      hakka"->"hak" in Section 4.5, noting that it was this document
      that caused the change.

   o  Replaced the section in Section 4.1 dealing with "mul"/"und" to
      include the subtags 'zxx' and 'mis', as well as the tag
      "i-default".  A normative reference to RFC 2277 was added.

   o  Added text to Section 3.5 clarifying that any modifications of a
      registration request must be sent to the <ietf-languages@iana.org>
      list before submission to IANA.

   o  Changed the ABNF for the record-jar format from using the LWSP
      production to use a folding whitespace production similar to obs-
      FWS in [RFC5234].  This effectively prevents unintentional blank
      lines inside a field.

   o  Clarified and revised text in Sections 3.3, 3.5, and 5.1 to
      clarify that the Language Subtag Reviewer sends the complete
      registration forms to IANA, that IANA extracts the record from the
      form, and that the forms must also be archived separately from the
      registry.




Phillips & Davis         Best Current Practice                 [Page 75]

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   o  Added text to Section 5 requiring IANA to send an announcement to
      an ietf-languages-announcements list whenever the registry is
      updated.

   o  Modification of the registry to use UTF-8 as its character
      encoding.  This also entails additional instructions to IANA and
      the Language Subtag Reviewer in the registration process.

   o  Modified the rules in Section 2.2.4 so that "exceptionally
      reserved" ISO 3166-1 codes other than 'UK' were included into the
      registry.  In particular, this allows the code 'EU' (European
      Union) to be used to form language tags or (more commonly) for
      applications that use the registry for region codes to reference
      this subtag.

   o  Modified the IANA considerations section (Section 5) to remove
      unnecessary normative [RFC2119] language.

9.  References

9.1.  Normative References

   [ISO15924]       International Organization for Standardization, "ISO
                    15924:2004.  Information and documentation -- Codes
                    for the representation of names of scripts",
                    January 2004.

   [ISO3166-1]      International Organization for Standardization, "ISO
                    3166-1:2006.  Codes for the representation of names
                    of countries and their subdivisions -- Part 1:
                    Country codes", November 2006.

   [ISO639-1]       International Organization for Standardization, "ISO
                    639-1:2002.  Codes for the representation of names
                    of languages -- Part 1: Alpha-2 code", July 2002.

   [ISO639-2]       International Organization for Standardization, "ISO
                    639-2:1998.  Codes for the representation of names
                    of languages -- Part 2: Alpha-3 code", October 1998.

   [ISO639-3]       International Organization for Standardization, "ISO
                    639-3:2007.  Codes for the representation of names
                    of languages - Part 3: Alpha-3 code for
                    comprehensive coverage of languages", February 2007.







Phillips & Davis         Best Current Practice                 [Page 76]

RFC 5646                     Language Tags                September 2009


   [ISO639-5]       International Organization for Standardization, "ISO
                    639-5:2008. Codes for the representation of names of
                    languages -- Part 5: Alpha-3 code for language
                    families and groups", May 2008.

   [ISO646]         International Organization for Standardization,
                    "ISO/IEC 646:1991, Information technology -- ISO
                    7-bit coded character set for information
                    interchange.", 1991.

   [RFC2026]        Bradner, S., "The Internet Standards Process --
                    Revision 3", BCP 9, RFC 2026, October 1996.

   [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                    Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2277]        Alvestrand, H., "IETF Policy on Character Sets and
                    Languages", BCP 18, RFC 2277, January 1998.

   [RFC3339]        Klyne, G., Ed. and C. Newman, "Date and Time on the
                    Internet: Timestamps", RFC 3339, July 2002.

   [RFC4647]        Phillips, A. and M. Davis, "Matching of Language
                    Tags", BCP 47, RFC 4647, September 2006.

   [RFC5226]        Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs",
                    BCP 26, RFC 5226, May 2008.

   [RFC5234]        Crocker, D. and P. Overell, "Augmented BNF for
                    Syntax Specifications: ABNF", STD 68, RFC 5234,
                    January 2008.

   [SpecialCasing]  The Unicode Consoritum, "Unicode Character Database,
                    Special Casing Properties", March 2008, <http://
                    unicode.org/Public/UNIDATA/SpecialCasing.txt>.

   [UAX14]          Freitag, A., "Unicode Standard Annex #14: Line
                    Breaking Properties", August 2006,
                    <http://www.unicode.org/reports/tr14/>.

   [UN_M.49]        Statistics Division, United Nations, "Standard
                    Country or Area Codes for Statistical Use", Revision
                    4 (United Nations publication, Sales No. 98.XVII.9,
                    June 1999.






Phillips & Davis         Best Current Practice                 [Page 77]

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   [Unicode]        Unicode Consortium, "The Unicode Consortium. The
                    Unicode Standard, Version 5.0, (Boston, MA, Addison-
                    Wesley, 2003. ISBN 0-321-49081-0)", January 2007.

9.2.  Informative References

   [CLDR]           "The Common Locale Data Repository Project",
                    <http://cldr.unicode.org>.

   [RFC1766]        Alvestrand, H., "Tags for the Identification of
                    Languages", RFC 1766, March 1995.

   [RFC2028]        Hovey, R. and S. Bradner, "The Organizations
                    Involved in the IETF Standards Process", BCP 11,
                    RFC 2028, October 1996.

   [RFC2046]        Freed, N. and N. Borenstein, "Multipurpose Internet
                    Mail Extensions (MIME) Part Two: Media Types",
                    RFC 2046, November 1996.

   [RFC2047]        Moore, K., "MIME (Multipurpose Internet Mail
                    Extensions) Part Three: Message Header Extensions
                    for Non-ASCII Text", RFC 2047, November 1996.

   [RFC2231]        Freed, N. and K. Moore, "MIME Parameter Value and
                    Encoded Word Extensions:
                    Character Sets, Languages, and Continuations",
                    RFC 2231, November 1997.

   [RFC2616]        Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                    Masinter, L., Leach, P., and T. Berners-Lee,
                    "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                    June 1999.

   [RFC2781]        Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
                    ISO 10646", RFC 2781, February 2000.

   [RFC3066]        Alvestrand, H., "Tags for the Identification of
                    Languages", RFC 3066, January 2001.

   [RFC3282]        Alvestrand, H., "Content Language Headers",
                    RFC 3282, May 2002.

   [RFC3552]        Rescorla, E. and B. Korver, "Guidelines for Writing
                    RFC Text on Security Considerations", BCP 72,
                    RFC 3552, July 2003.





Phillips & Davis         Best Current Practice                 [Page 78]

RFC 5646                     Language Tags                September 2009


   [RFC3629]        Yergeau, F., "UTF-8, a transformation format of ISO
                    10646", STD 63, RFC 3629, November 2003.

   [RFC4645]        Ewell, D., "Initial Language Subtag Registry",
                    RFC 4645, September 2006.

   [RFC4646]        Phillips, A. and M. Davis, "Tags for Identifying
                    Languages", BCP 47, RFC 4646, September 2006.

   [RFC5645]        Ewell, D., Ed., "Update to the Language Subtag
                    Registry", September 2009.

   [UTS35]          Davis, M., "Unicode Technical Standard #35: Locale
                    Data Markup Language (LDML)", December 2007,
                    <http://www.unicode.org/reports/tr35/>.

   [iso639.prin]    ISO 639 Joint Advisory Committee, "ISO 639 Joint
                    Advisory Committee:  Working principles for ISO 639
                    maintenance", March 2000, <http://www.loc.gov/
                    standards/iso639-2/iso639jac_n3r.html>.

   [record-jar]     Raymond, E., "The Art of Unix Programming", 2003,
                    <urn:isbn:0-13-142901-9>.




























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Appendix A.  Examples of Language Tags (Informative)

   Simple language subtag:

      de (German)

      fr (French)

      ja (Japanese)

      i-enochian (example of a grandfathered tag)

   Language subtag plus Script subtag:

      zh-Hant (Chinese written using the Traditional Chinese script)

      zh-Hans (Chinese written using the Simplified Chinese script)

      sr-Cyrl (Serbian written using the Cyrillic script)

      sr-Latn (Serbian written using the Latin script)

   Extended language subtags and their primary language subtag
   counterparts:

      zh-cmn-Hans-CN (Chinese, Mandarin, Simplified script, as used in
      China)

      cmn-Hans-CN (Mandarin Chinese, Simplified script, as used in
      China)

      zh-yue-HK (Chinese, Cantonese, as used in Hong Kong SAR)

      yue-HK (Cantonese Chinese, as used in Hong Kong SAR)

   Language-Script-Region:

      zh-Hans-CN (Chinese written using the Simplified script as used in
      mainland China)

      sr-Latn-RS (Serbian written using the Latin script as used in
      Serbia)









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   Language-Variant:

      sl-rozaj (Resian dialect of Slovenian)

      sl-rozaj-biske (San Giorgio dialect of Resian dialect of
      Slovenian)

      sl-nedis (Nadiza dialect of Slovenian)

   Language-Region-Variant:

      de-CH-1901 (German as used in Switzerland using the 1901 variant
      [orthography])

      sl-IT-nedis (Slovenian as used in Italy, Nadiza dialect)

   Language-Script-Region-Variant:

      hy-Latn-IT-arevela (Eastern Armenian written in Latin script, as
      used in Italy)

   Language-Region:

      de-DE (German for Germany)

      en-US (English as used in the United States)

      es-419 (Spanish appropriate for the Latin America and Caribbean
      region using the UN region code)

   Private use subtags:

      de-CH-x-phonebk

      az-Arab-x-AZE-derbend

   Private use registry values:

      x-whatever (private use using the singleton 'x')

      qaa-Qaaa-QM-x-southern (all private tags)

      de-Qaaa (German, with a private script)

      sr-Latn-QM (Serbian, Latin script, private region)

      sr-Qaaa-RS (Serbian, private script, for Serbia)




Phillips & Davis         Best Current Practice                 [Page 81]

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   Tags that use extensions (examples ONLY -- extensions MUST be defined
   by revision or update to this document, or by RFC):

      en-US-u-islamcal

      zh-CN-a-myext-x-private

      en-a-myext-b-another

   Some Invalid Tags:

      de-419-DE (two region tags)

      a-DE (use of a single-character subtag in primary position; note
      that there are a few grandfathered tags that start with "i-" that
      are valid)

      ar-a-aaa-b-bbb-a-ccc (two extensions with same single-letter
      prefix)

Appendix B.  Examples of Registration Forms

   LANGUAGE SUBTAG REGISTRATION FORM

   1. Name of requester: Han Steenwijk
   2. E-mail address of requester: han.steenwijk @ unipd.it
   3. Record Requested:

   Type:        variant
   Subtag:      biske
   Description: The San Giorgio dialect of Resian
   Description: The Bila dialect of Resian
   Prefix:      sl-rozaj
   Comments:    The dialect of San Giorgio/Bila is one of the
      four major local dialects of Resian

   4. Intended meaning of the subtag:

   The local variety of Resian as spoken in San Giorgio/Bila

   5. Reference to published description of the language (book or
   article):

    -- Jan I.N. Baudouin de Courtenay - Opyt fonetiki rez'janskich
   govorov, Varsava - Peterburg: Vende - Kozancikov, 1875.






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   LANGUAGE SUBTAG REGISTRATION FORM

   1. Name of requester: Jaska Zedlik
   2. E-mail address of requester: jz53 @ zedlik.com
   3. Record Requested:

   Type:   variant
   Subtag: tarask
   Description: Belarusian in Taraskievica orthography
   Prefix: be
   Comments: The subtag represents Branislau Taraskievic's Belarusian
     orthography as published in "Bielaruski klasycny pravapis" by
     Juras Buslakou, Vincuk Viacorka, Zmicier Sanko, and Zmicier Sauka
     (Vilnia-Miensk 2005).

   4. Intended meaning of the subtag:

   The subtag is intended to represent the Belarusian orthography as
   published in "Bielaruski klasycny pravapis" by Juras Buslakou, Vincuk
   Viacorka, Zmicier Sanko, and Zmicier Sauka (Vilnia-Miensk 2005).

   5. Reference to published description of the language (book or
   article):

   Taraskievic, Branislau. Bielaruskaja gramatyka dla skol. Vilnia: Vyd.
   "Bielaruskaha kamitetu", 1929, 5th edition.

   Buslakou, Juras; Viacorka, Vincuk; Sanko, Zmicier; Sauka, Zmicier.
   Bielaruski klasycny pravapis. Vilnia-Miensk, 2005.

   6. Any other relevant information:

   Belarusian in Taraskievica orthography became widely used, especially
   in Belarusian-speaking Internet segment, but besides this some books
   and newspapers are also printed using this orthography of Belarusian.

Appendix C.  Acknowledgements

   Any list of contributors is bound to be incomplete; please regard the
   following as only a selection from the group of people who have
   contributed to make this document what it is today.

   The contributors to RFC 4646, RFC 4647, RFC 3066, and RFC 1766, the
   precursors of this document, made enormous contributions directly or
   indirectly to this document and are generally responsible for the
   success of language tags.





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   The following people contributed to this document:

   Stephane Bortzmeyer, Karen Broome, Peter Constable, John Cowan,
   Martin Duerst, Frank Ellerman, Doug Ewell, Deborah Garside, Marion
   Gunn, Alfred Hoenes, Kent Karlsson, Chris Newman, Randy Presuhn,
   Stephen Silver, Shawn Steele, and many, many others.

   Very special thanks must go to Harald Tveit Alvestrand, who
   originated RFCs 1766 and 3066, and without whom this document would
   not have been possible.

   Special thanks go to Michael Everson, who served as the Language Tag
   Reviewer for almost the entire RFC 1766/RFC 3066 period, as well as
   the Language Subtag Reviewer since the adoption of RFC 4646.

   Special thanks also go to Doug Ewell, for his production of the first
   complete subtag registry, his work to support and maintain new
   registrations, and his careful editorship of both RFC 4645 and
   [RFC5645].

Authors' Addresses

   Addison Phillips (editor)
   Lab126

   EMail: addison@inter-locale.com
   URI:   http://www.inter-locale.com


   Mark Davis (editor)
   Google

   EMail: markdavis@google.com


















Phillips & Davis         Best Current Practice                 [Page 84]
=========================================================================





Network Working Group                                   A. Phillips, Ed.
Request for Comments: 4647                                   Yahoo! Inc.
BCP: 47                                                    M. Davis, Ed.
Obsoletes: 3066                                                   Google
Category: Best Current Practice                           September 2006


                       Matching of Language Tags

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a syntax, called a "language-range", for
   specifying items in a user's list of language preferences.  It also
   describes different mechanisms for comparing and matching these to
   language tags.  Two kinds of matching mechanisms, filtering and
   lookup, are defined.  Filtering produces a (potentially empty) set of
   language tags, whereas lookup produces a single language tag.
   Possible applications include language negotiation or content
   selection.  This document, in combination with RFC 4646, replaces RFC
   3066, which replaced RFC 1766.





















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Table of Contents

   1. Introduction ....................................................3
   2. The Language Range ..............................................3
      2.1. Basic Language Range .......................................4
      2.2. Extended Language Range ....................................4
      2.3. The Language Priority List .................................5
   3. Types of Matching ...............................................6
      3.1. Choosing a Matching Scheme .................................6
      3.2. Implementation Considerations ..............................7
      3.3. Filtering ..................................................8
           3.3.1. Basic Filtering .....................................9
           3.3.2. Extended Filtering .................................10
      3.4. Lookup ....................................................12
           3.4.1. Default Values .....................................14
   4. Other Considerations ...........................................15
      4.1. Choosing Language Ranges ..................................15
      4.2. Meaning of Language Tags and Ranges .......................16
      4.3. Considerations for Private-Use Subtags ....................17
      4.4. Length Considerations for Language Ranges .................17
   5. Security Considerations ........................................17
   6. Character Set Considerations ...................................17
   7. References .....................................................18
      7.1. Normative References ......................................18
      7.2. Informative References ....................................18
   Appendix A. Acknowledgements ......................................19

























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1.  Introduction

   Human beings on our planet have, past and present, used a number of
   languages.  There are many reasons why one would want to identify the
   language used when presenting or requesting information.

   Applications, protocols, or specifications that use language
   identifiers, such as the language tags defined in [RFC4646],
   sometimes need to match language tags to a user's language
   preferences.

   This document defines a syntax (called a language range (Section 2))
   for specifying items in the user's list of language preferences
   (called a language priority list (Section 2.3)), as well as several
   schemes for selecting or filtering sets of language tags by comparing
   the language tags to the user's preferences.  Applications,
   protocols, or specifications will have varying needs and requirements
   that affect the choice of a suitable matching scheme.

   This document describes how to indicate a user's preferences using
   language ranges, three schemes for matching these ranges to a set of
   language tags, and the various practical considerations that apply to
   implementing and using these schemes.

   This document, in combination with [RFC4646], replaces [RFC3066],
   which replaced [RFC1766].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  The Language Range

   Language tags [RFC4646] are used to help identify languages, whether
   spoken, written, signed, or otherwise signaled, for the purpose of
   communication.  Applications, protocols, or specifications that use
   language tags are often faced with the problem of identifying sets of
   content that share certain language attributes.  For example,
   HTTP/1.1 [RFC2616] describes one such mechanism in its discussion of
   the Accept-Language header (Section 14.4), which is used when
   selecting content from servers based on the language of that content.

   It is, thus, useful to have a mechanism for identifying sets of
   language tags that share specific attributes.  This allows users to
   select or filter the language tags based on specific requirements.
   Such an identifier is called a "language range".





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   There are different types of language range, whose specific
   attributes vary according to their application.  Language ranges are
   similar to language tags: they consist of a sequence of subtags
   separated by hyphens.  In a language range, each subtag MUST either
   be a sequence of ASCII alphanumeric characters or the single
   character '*' (%x2A, ASTERISK).  The character '*' is a "wildcard"
   that matches any sequence of subtags.  The meaning and uses of
   wildcards vary according to the type of language range.

   Language tags and thus language ranges are to be treated as case-
   insensitive: there exist conventions for the capitalization of some
   of the subtags, but these MUST NOT be taken to carry meaning.
   Matching of language tags to language ranges MUST be done in a case-
   insensitive manner.

2.1.  Basic Language Range

   A "basic language range" has the same syntax as an [RFC3066] language
   tag or is the single character "*".  The basic language range was
   originally described by HTTP/1.1 [RFC2616] and later [RFC3066].  It
   is defined by the following ABNF [RFC4234]:

   language-range   = (1*8ALPHA *("-" 1*8alphanum)) / "*"
   alphanum         = ALPHA / DIGIT

   A basic language range differs from the language tags defined in
   [RFC4646] only in that there is no requirement that it be "well-
   formed" or be validated against the IANA Language Subtag Registry.
   Such ill-formed ranges will probably not match anything.  Note that
   the ABNF [RFC4234] in [RFC2616] is incorrect, since it disallows the
   use of digits anywhere in the 'language-range' (see [RFC2616errata]).

2.2.  Extended Language Range

   Occasionally, users will wish to select a set of language tags based
   on the presence of specific subtags.  An "extended language range"
   describes a user's language preference as an ordered sequence of
   subtags.  For example, a user might wish to select all language tags
   that contain the region subtag 'CH' (Switzerland).  Extended language
   ranges are useful for specifying a particular sequence of subtags
   that appear in the set of matching tags without having to specify all
   of the intervening subtags.

   An extended language range can be represented by the following ABNF:

   extended-language-range = (1*8ALPHA / "*")
                             *("-" (1*8alphanum / "*"))




Phillips & Davis         Best Current Practice                  [Page 4]

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   The wildcard subtag '*' can occur in any position in the extended
   language range, where it matches any sequence of subtags that might
   occur in that position in a language tag.  However, wildcards outside
   the first position are ignored by Extended Filtering (see Section
   3.2.2).  The use or absence of one or more wildcards cannot be taken
   to imply that a certain number of subtags will appear in the matching
   set of language tags.

2.3.  The Language Priority List

   A user's language preferences will often need to specify more than
   one language range, and thus users often need to specify a
   prioritized list of language ranges in order to best reflect their
   language preferences.  This is especially true for speakers of
   minority languages.  A speaker of Breton in France, for example, can
   specify "br" followed by "fr", meaning that if Breton is available,
   it is preferred, but otherwise French is the best alternative.  It
   can get more complex: a different user might want to fall back from
   Skolt Sami to Northern Sami to Finnish.

   A "language priority list" is a prioritized or weighted list of
   language ranges.  One well-known example of such a list is the
   "Accept-Language" header defined in RFC 2616 [RFC2616] (see Section
   14.4) and RFC 3282 [RFC3282].

   The various matching operations described in this document include
   considerations for using a language priority list.  This document
   does not define the syntax for a language priority list; defining
   such a syntax is the responsibility of the protocol, application, or
   specification that uses it.  When given as examples in this document,
   language priority lists will be shown as a quoted sequence of ranges
   separated by commas, like this: "en, fr, zh-Hant" (which is read
   "English before French before Chinese as written in the Traditional
   script").

   A simple list of ranges is considered to be in descending order of
   priority.  Other language priority lists provide "quality weights"
   for the language ranges in order to specify the relative priority of
   the user's language preferences.  An example of this is the use of
   "q" values in the syntax of the "Accept-Language" header (defined in
   [RFC2616], Section 14.4, and [RFC3282]).










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3.  Types of Matching

   Matching language ranges to language tags can be done in many
   different ways.  This section describes three such matching schemes,
   as well as the considerations for choosing between them.  Protocols
   and specifications requiring conformance to this specification MUST
   clearly indicate the particular mechanism used in selecting or
   matching language tags.

   There are two types of matching scheme in this document.  A matching
   scheme that produces zero or more matching language tags is called
   "filtering".  A matching scheme that produces exactly one match for a
   given request is called "lookup".

3.1.  Choosing a Matching Scheme

   Applications, protocols, and specifications are faced with the
   decision of what type of matching to use.  Sometimes, different
   styles of matching are suited to different kinds of processing within
   a particular application or protocol.

   This document describes three matching schemes:

   1.  Basic Filtering (Section 3.3.1) matches a language priority list
       consisting of basic language ranges (Section 2.1) to sets of
       language tags.

   2.  Extended Filtering (Section 3.3.2) matches a language priority
       list consisting of extended language ranges (Section 2.2) to sets
       of language tags.

   3.  Lookup (Section 3.4) matches a language priority list consisting
       of basic language ranges to sets of language tags to find the one
       exact language tag that best matches the range.

   Filtering can be used to produce a set of results (such as a
   collection of documents) by comparing the user's preferences to a set
   of language tags.  For example, when performing a search, filtering
   can be used to limit the results to items tagged as being in the
   French language.  Filtering can also be used when deciding whether to
   perform a language-sensitive process on some content.  For example, a
   process might cause paragraphs whose language tag matched the
   language range "nl" (Dutch) to be displayed in italics within a
   document.

   Lookup produces the single result that best matches the user's
   preferences from the list of available tags, so it is useful in cases
   in which a single item is required (and for which only a single item



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   can be returned).  For example, if a process were to insert a human-
   readable error message into a protocol header, it might select the
   text based on the user's language priority list.  Since the process
   can return only one item, it is forced to choose a single item and it
   has to return some item, even if none of the content's language tags
   match the language priority list supplied by the user.

3.2.  Implementation Considerations

   Language tag matching is a tool, and does not by itself specify a
   complete procedure for the use of language tags.  Such procedures are
   intimately tied to the application protocol in which they occur.
   When specifying a protocol operation using matching, the protocol
   MUST specify:

   o  Which type(s) of language tag matching it uses

   o  Whether the operation returns a single result (lookup) or a
      possibly empty set of results (filtering)

   o  For lookup, what the default item is (or the sequence of
      operations or configuration information used to determine the
      default) when no matching tag is found.  For instance, a protocol
      might define the result as failure of the operation, an empty
      value, returning some protocol defined or implementation defined
      default, or returning i-default [RFC2277].

   Applications, protocols, and specifications are not required to
   validate or understand any of the semantics of the language tags or
   ranges or of the subtags in them, nor do they require access to the
   IANA Language Subtag Registry (see Section 3 in [RFC4646]).  This
   simplifies implementation.

   However, designers of applications, protocols, or specifications are
   encouraged to use the information from the IANA Language Subtag
   Registry to support canonicalizing language tags and ranges in order
   to map grandfathered and obsolete tags or subtags into modern
   equivalents.

   Applications, protocols, or specifications that canonicalize ranges
   MUST either perform matching operations with both the canonical and
   original (unmodified) form of the range or MUST also canonicalize
   each tag for the purposes of comparison.








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   Note that canonicalizing language ranges makes certain operations
   impossible.  For example, an implementation that canonicalizes the
   language range "art-lojban" (artificial language, lojban variant) to
   use the more modern "jbo" (Lojban) cannot be used to select just the
   items with the older tag.

   Applications, protocols, or specifications that use basic ranges
   might sometimes receive extended language ranges instead.  An
   application, protocol, or specification MUST choose to a) map
   extended language ranges to basic ranges using the algorithm below,
   b) reject any extended language ranges in the language priority list
   that are not valid basic language ranges, or c) treat each extended
   language range as if it were a basic language range, which will have
   the same result as ignoring them, since these ranges will not match
   any valid language tags.

   An extended language range is mapped to a basic language range as
   follows: if the first subtag is a '*' then the entire range is
   treated as "*", otherwise each wildcard subtag is removed.  For
   example, the extended language range "en-*-US" maps to "en-US"
   (English, United States).

   Applications, protocols, or specifications, in addressing their
   particular requirements, can offer pre-processing or configuration
   options.  For example, an implementation could allow a user to
   associate or map a particular language range to a different value.
   Such a user might wish to associate the language range subtags 'nn'
   (Nynorsk Norwegian) and 'nb' (Bokmal Norwegian) with the more general
   subtag 'no' (Norwegian).  Or perhaps a user would want to associate
   requests for the range "zh-Hans" (Chinese as written in the
   Simplified script) with content bearing the language tag "zh-CN"
   (Chinese as used in China, where the Simplified script is
   predominant).  Documentation on how the ranges or tags are altered,
   prioritized, or compared in the subsequent match in such an
   implementation will assist users in making these types of
   configuration choices.

3.3.  Filtering

   Filtering is used to select the set of language tags that matches a
   given language priority list.  It is called "filtering" because this
   set might contain no items at all or it might return an arbitrarily
   large number of matching items: as many items as match the language
   priority list, thus "filtering out" the non-matching items.

   In filtering, each language range represents the least specific
   language tag (that is, the language tag with fewest number of
   subtags) that is an acceptable match.  All of the language tags in



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   the matching set of tags will have an equal or greater number of
   subtags than the language range.  Every non-wildcard subtag in the
   language range will appear in every one of the matching language
   tags.  For example, if the language priority list consists of the
   range "de-CH" (German as used in Switzerland), one might see tags
   such as "de-CH-1996" (German as used in Switzerland, orthography of
   1996) but one will never see a tag such as "de" (because the 'CH'
   subtag is missing).

   If the language priority list (see Section 2.3) contains more than
   one range, the content returned is typically ordered in descending
   level of preference, but it MAY be unordered, according to the needs
   of the application or protocol.

   Some examples of applications where filtering might be appropriate
   include:

   o  Applying a style to sections of a document in a particular set of
      languages.

   o  Displaying the set of documents containing a particular set of
      keywords written in a specific set of languages.

   o  Selecting all email items written in a specific set of languages.

   o  Selecting audio files spoken in a particular language.

   Filtering seems to imply that there is a semantic relationship
   between language tags that share the same prefix.  While this is
   often the case, it is not always true: the language tags that match a
   specific language range do not necessarily represent mutually
   intelligible languages.

3.3.1.  Basic Filtering

   Basic filtering compares basic language ranges to language tags.
   Each basic language range in the language priority list is considered
   in turn, according to priority.  A language range matches a
   particular language tag if, in a case-insensitive comparison, it
   exactly equals the tag, or if it exactly equals a prefix of the tag
   such that the first character following the prefix is "-".  For
   example, the language-range "de-de" (German as used in Germany)
   matches the language tag "de-DE-1996" (German as used in Germany,
   orthography of 1996), but not the language tags "de-Deva" (German as
   written in the Devanagari script) or "de-Latn-DE" (German, Latin
   script, as used in Germany).





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   The special range "*" in a language priority list matches any tag.  A
   protocol that uses language ranges MAY specify additional rules about
   the semantics of "*"; for instance, HTTP/1.1 [RFC2616] specifies that
   the range "*" matches only languages not matched by any other range
   within an "Accept-Language" header.

   Basic filtering is identical to the type of matching described in
   [RFC3066], Section 2.5 (Language-range).

3.3.2.  Extended Filtering

   Extended filtering compares extended language ranges to language
   tags.  Each extended language range in the language priority list is
   considered in turn, according to priority.  A language range matches
   a particular language tag if each respective list of subtags matches.
   To determine a match:

   1.  Split both the extended language range and the language tag being
       compared into a list of subtags by dividing on the hyphen (%x2D)
       character.  Two subtags match if either they are the same when
       compared case-insensitively or the language range's subtag is the
       wildcard '*'.

   2.  Begin with the first subtag in each list.  If the first subtag in
       the range does not match the first subtag in the tag, the overall
       match fails.  Otherwise, move to the next subtag in both the
       range and the tag.

   3.  While there are more subtags left in the language range's list:

       A.  If the subtag currently being examined in the range is the
           wildcard ('*'), move to the next subtag in the range and
           continue with the loop.

       B.  Else, if there are no more subtags in the language tag's
           list, the match fails.

       C.  Else, if the current subtag in the range's list matches the
           current subtag in the language tag's list, move to the next
           subtag in both lists and continue with the loop.

       D.  Else, if the language tag's subtag is a "singleton" (a single
           letter or digit, which includes the private-use subtag 'x')
           the match fails.

       E.  Else, move to the next subtag in the language tag's list and
           continue with the loop.




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   4.  When the language range's list has no more subtags, the match
       succeeds.

   Subtags not specified, including those at the end of the language
   range, are thus treated as if assigned the wildcard value '*'.  Much
   like basic filtering, extended filtering selects content with
   arbitrarily long tags that share the same initial subtags as the
   language range.  In addition, extended filtering selects language
   tags that contain any intermediate subtags not specified in the
   language range.  For example, the extended language range "de-*-DE"
   (or its synonym "de-DE") matches all of the following tags:

      de-DE (German, as used in Germany)

      de-de (German, as used in Germany)

      de-Latn-DE (Latin script)

      de-Latf-DE (Fraktur variant of Latin script)

      de-DE-x-goethe (private-use subtag)

      de-Latn-DE-1996 (orthography of 1996)

      de-Deva-DE (Devanagari script)

   The same range does not match any of the following tags for the
   reasons shown:

      de (missing 'DE')

      de-x-DE (singleton 'x' occurs before 'DE')

      de-Deva ('Deva' not equal to 'DE')

   Note: [RFC4646] defines each type of subtag (language, script,
   region, and so forth) according to position, size, and content.  This
   means that subtags in a language range can only match specific types
   of subtags in a language tag.  For example, a subtag such as 'Latn'
   is always a script subtag (unless it follows a singleton) while a
   subtag such as 'nedis' can only match the equivalent variant subtag.
   Two-letter subtags in the initial position have a different type
   (language) than two-letter subtags in later positions (region).  This
   is the reason why a wildcard in the extended language range is
   significant in the first position but is ignored in all other
   positions.





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3.4.  Lookup

   Lookup is used to select the single language tag that best matches
   the language priority list for a given request.  When performing
   lookup, each language range in the language priority list is
   considered in turn, according to priority.  By contrast with
   filtering, each language range represents the most specific tag that
   is an acceptable match.  The first matching tag found, according to
   the user's priority, is considered the closest match and is the item
   returned.  For example, if the language range is "de-ch", a lookup
   operation can produce content with the tags "de" or "de-CH" but never
   content with the tag "de-CH-1996".  If no language tag matches the
   request, the "default" value is returned.

   For example, if an application inserts some dynamic content into a
   document, returning an empty string if there is no exact match is not
   an option.  Instead, the application "falls back" until it finds a
   matching language tag associated with a suitable piece of content to
   insert.  Some applications of lookup include:

   o  Selection of a template containing the text for an automated email
      response.

   o  Selection of an item containing some text for inclusion in a
      particular Web page.

   o  Selection of a string of text for inclusion in an error log.

   o  Selection of an audio file to play as a prompt in a phone system.

   In the lookup scheme, the language range is progressively truncated
   from the end until a matching language tag is located.  Single letter
   or digit subtags (including both the letter 'x', which introduces
   private-use sequences, and the subtags that introduce extensions) are
   removed at the same time as their closest trailing subtag.  For
   example, starting with the range "zh-Hant-CN-x-private1-private2"
   (Chinese, Traditional script, China, two private-use tags) the lookup
   progressively searches for content as shown below:

   Example of a Lookup Fallback Pattern

   Range to match: zh-Hant-CN-x-private1-private2
   1. zh-Hant-CN-x-private1-private2
   2. zh-Hant-CN-x-private1
   3. zh-Hant-CN
   4. zh-Hant
   5. zh
   6. (default)



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   This fallback behavior allows some flexibility in finding a match.
   Without fallback, the default content would be returned immediately
   if exactly matching content is unavailable.  With fallback, a result
   more closely matching the user request can be provided.

   Extensions and unrecognized private-use subtags might be unrelated to
   a particular application of lookup.  Since these subtags come at the
   end of the subtag sequence, they are removed first during the
   fallback process and usually pose no barrier to interoperability.
   However, an implementation MAY remove these from ranges prior to
   performing the lookup (provided the implementation also removes them
   from the tags being compared).  Such modification is internal to the
   implementation and applications, protocols, or specifications SHOULD
   NOT remove or modify subtags in content that they return or forward,
   because this removes information that can be used elsewhere.

   The special language range "*" matches any language tag.  In the
   lookup scheme, this range does not convey enough information by
   itself to determine which language tag is most appropriate, since it
   matches everything.  If the language range "*" is followed by other
   language ranges, it is skipped.  If the language range "*" is the
   only one in the language priority list or if no other language range
   follows, the default value is computed and returned.

   In some cases, the language priority list can contain one or more
   extended language ranges (as, for example, when the same language
   priority list is used as input for both lookup and filtering
   operations).  Wildcard values in an extended language range normally
   match any value that can occur in that position in a language tag.
   Since only one item can be returned for any given lookup request,
   wildcards in a language range have to be processed in a consistent
   manner or the same request will produce widely varying results.
   Applications, protocols, or specifications that accept extended
   language ranges MUST define which item is returned when more than one
   item matches the extended language range.

   For example, an implementation could map the extended language ranges
   to basic ranges.  Another possibility would be for an implementation
   to return the matching tag that is first in ASCII-order.  If the
   language range were "*-CH" ('CH' represents Switzerland) and the set
   of tags included "de-CH" (German as used in Switzerland), "fr-CH"
   (French, Switzerland), and "it-CH" (Italian, Switzerland), then the
   tag "de-CH" would be returned.








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3.4.1.  Default Values

   Each application, protocol, or specification that uses lookup MUST
   define the defaulting behavior when no tag matches the language
   priority list.  What this action consists of strongly depends on how
   lookup is being applied.  Some examples of defaulting behavior
   include:

   o  return an item with no language tag or an item of a non-linguistic
      nature, such as an image or sound

   o  return a null string as the language tag value, in cases where the
      protocol permits the empty value (see, for example, "xml:lang" in
      [XML10])

   o  return a particular language tag designated for the operation

   o  return the language tag "i-default" (see [RFC2277])

   o  return an error condition or error message

   o  return a list of available languages for the user to select from

   When performing lookup using a language priority list, the
   progressive search MUST process each language range in the list
   before seeking or calculating the default.

   The default value MAY be calculated or include additional searching
   or matching.  Applications, protocols, or specifications can specify
   different ways in which users can specify or override the defaults.

   One common way to provide for a default is to allow a specific
   language range to be set as the default for a specific type of
   request.  If this approach is chosen, this language range MUST be
   treated as if it were appended to the end of the language priority
   list as a whole, rather than after each item in the language priority
   list.  The application, protocol, or specification MUST also define
   the defaulting behavior if that search fails to find a matching tag
   or item.

   For example, if a particular user's language priority list is "fr-FR,
   zh-Hant" (French as used in France followed by Chinese as written in
   the Traditional script) and the program doing the matching had a
   default language range of "ja-JP" (Japanese as used in Japan), then
   the program searches as follows:






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   1. fr-FR
   2. fr
   3. zh-Hant // next language
   4. zh
   5. ja-JP   // now searching for the default content
   6. ja
   7. (implementation defined default)

4.  Other Considerations

   When working with language ranges and matching schemes, there are
   some additional points that can influence the choice of either.

4.1.  Choosing Language Ranges

   Users indicate their language preferences via the choice of a
   language range or the list of language ranges in a language priority
   list.  The type of matching affects what the best choice is for a
   user.

   Most matching schemes make no attempt to process the semantic meaning
   of the subtags.  The language range is compared, in a case-
   insensitive manner, to each language tag being matched, using basic
   string processing.  Users SHOULD select language ranges that are
   well-formed, valid language tags according to [RFC4646] (substituting
   wildcards as appropriate in extended language ranges).

   Applications are encouraged to canonicalize language tags and ranges
   by using the Preferred-Value from the IANA Language Subtag Registry
   for tags or subtags that have been deprecated.  If the user is
   working with content that might use the older form, the user might
   want to include both the new and old forms in a language priority
   list.  For example, the tag "art-lojban" is deprecated.  The subtag
   'jbo' is supposed to be used instead, so the user might use it to
   form the language range.  Or the user might include both in a
   language priority list: "jbo, art-lojban".

   Users SHOULD avoid subtags that add no distinguishing value to a
   language range.  When filtering, the fewer the number of subtags that
   appear in the language range, the more content the range will
   probably match, while in lookup unnecessary subtags can cause
   "better", more-specific content to be skipped in favor of less
   specific content.  For example, the range "de-Latn-DE" returns
   content tagged "de" instead of content tagged "de-DE", even though
   the latter is probably a better match.






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   Whether a subtag adds distinguishing value can depend on the context
   of the request.  For example, a user who reads both Simplified and
   Traditional Chinese, but who prefers Simplified, might use the range
   "zh" for filtering (matching all items that user can read) but
   "zh-Hans" for lookup (making sure that user gets the preferred form
   if it's available, but the fallback to "zh" will still work).  On the
   other hand, content in this case ought to be labeled as "zh-Hans" (or
   "zh-Hant" if that applies) for filtering, while for lookup, if there
   is either "zh-Hans" content or "zh-Hant" content, one of them (the
   one considered 'default') also ought to be made available with the
   simple "zh".  Note that the user can create a language priority list
   "zh-Hans, zh" that delivers the best possible results for both
   schemes.  If the user cannot be sure which scheme is being used (or
   if more than one might be applied to a given request), the user
   SHOULD specify the most specific (largest number of subtags) range
   first and then supply shorter prefixes later in the list to ensure
   that filtering returns a complete set of tags.

   Many languages are written predominantly in a single script.  This is
   usually recorded in the Suppress-Script field in that language
   subtag's registry entry.  For these languages, script subtags SHOULD
   NOT be used to form a language range.  Thus, the language range
   "en-Latn" is inappropriate in most cases (because the vast majority
   of English documents are written in the Latin script and thus the
   'en' language subtag has a Suppress-Script field for 'Latn' in the
   registry).

   When working with tags and ranges, note that extensions and most
   private-use subtags are orthogonal to language tag matching, in that
   they specify additional attributes of the text not related to the
   goals of most matching schemes.  Users SHOULD avoid using these
   subtags in language ranges, since they interfere with the selection
   of available content.  When used in language tags (as opposed to
   ranges), these subtags normally do not interfere with filtering
   (Section 3), since they appear at the end of the tag and will match
   all prefixes.  Lookup (Section 3.4) implementations are advised to
   ignore unrecognized private-use and extension subtags when performing
   language tag fallback.

4.2.  Meaning of Language Tags and Ranges

   Selecting language tags using language ranges requires some
   understanding by users of what they are selecting.  The meanings of
   the various subtags in a language range are identical to their
   meanings in a language tag (see Section 4.2 in [RFC4646]), with the
   addition that the wildcard "*" represents any matching sequence of
   values.




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4.3.  Considerations for Private-Use Subtags

   Private agreement is necessary between the parties that intend to use
   or exchange language tags that contain private-use subtags.  Great
   caution SHOULD be used in employing private-use subtags in content or
   protocols intended for general use.  Private-use subtags are simply
   useless for information exchange without prior arrangement.

   The value and semantic meaning of private-use tags and of the subtags
   used within such a language tag are not defined.  Matching private-
   use tags using language ranges or extended language ranges can result
   in unpredictable content being returned.

4.4.  Length Considerations for Language Ranges

   Language ranges are very similar to language tags in terms of content
   and usage.  The same types of restrictions on length that can be
   applied to language tags can also be applied to language ranges.  See
   [RFC4646] Section 4.3 (Length Considerations).

5.  Security Considerations

   Language ranges used in content negotiation might be used to infer
   the nationality of the sender, and thus identify potential targets
   for surveillance.  In addition, unique or highly unusual language
   ranges or combinations of language ranges might be used to track a
   specific individual's activities.

   This is a special case of the general problem that anything you send
   is visible to the receiving party.  It is useful to be aware that
   such concerns can exist in some cases.

   The evaluation of the exact magnitude of the threat, and any possible
   countermeasures, is left to each application or protocol.

6.  Character Set Considerations

   Language tags permit only the characters A-Z, a-z, 0-9, and HYPHEN-
   MINUS (%x2D).  Language ranges also use the character ASTERISK
   (%x2A).  These characters are present in most character sets, so
   presentation or exchange of language tags or ranges should not be
   constrained by character set issues.









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7.  References

7.1.  Normative References

   [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2277]       Alvestrand, H., "IETF Policy on Character Sets and
                   Languages", BCP 18, RFC 2277, January 1998.

   [RFC4234]       Crocker, D., Ed. and P. Overell, "Augmented BNF for
                   Syntax Specifications: ABNF", RFC 4234, October 2005.

   [RFC4646]       Phillips, A., Ed., and M. Davis, Ed., "Tags for
                   Identifying Languages", BCP 47, RFC 4646, September
                   2006.

7.2.  Informative References

   [RFC1766]       Alvestrand, H., "Tags for the Identification of
                   Languages", RFC 1766, March 1995.

   [RFC2616]       Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                   Masinter, L., Leach, P., and T. Berners-Lee,
                   "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                   June 1999.

   [RFC2616errata] IETF, "HTTP/1.1 Specification Errata", October 2004,
                   <http://purl.org/NET/http-errata>.

   [RFC3066]       Alvestrand, H., "Tags for the Identification of
                   Languages", BCP 47, RFC 3066, January 2001.

   [RFC3282]       Alvestrand, H., "Content Language Headers", RFC 3282,
                   May 2002.

   [XML10]         Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
                   and F. Yergeau, "Extensible Markup Language (XML) 1.0
                   (Third Edition)", World Wide Web Consortium
                   Recommendation, February 2004,
                   <http://www.w3.org/TR/REC-xml>.










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Appendix A.  Acknowledgements

   Any list of contributors is bound to be incomplete; please regard the
   following as only a selection from the group of people who have
   contributed to make this document what it is today.

   The contributors to [RFC1766] and [RFC3066], each of which was a
   precursor to this document, contributed greatly to the development of
   language tag matching, and, in particular, the basic language range
   and the basic matching scheme.  This document was originally part of
   [RFC4646], but was split off before that document's completion.
   Thus, directly or indirectly, those acknowledged in [RFC4646] also
   had a hand in the development of this document, and work done prior
   to the split is acknowledged in that document.

   The following people (in alphabetical order by family name)
   contributed to this document:

   Harald Alvestrand, Stephane Bortzmeyer, Jeremy Carroll, Peter
   Constable, John Cowan, Mark Crispin, Martin Duerst, Frank Ellermann,
   Doug Ewell, Debbie Garside, Marion Gunn, Jon Hanna, Kent Karlsson,
   Erkki Kolehmainen, Jukka Korpela, Ira McDonald, M. Patton, Randy
   Presuhn, Eric van der Poel, Markus Scherer, Misha Wolf, and many,
   many others.

   Very special thanks must go to Harald Tveit Alvestrand, who
   originated RFCs 1766 and 3066, and without whom this document would
   not have been possible.

Authors' Addresses

   Addison Phillips (Editor)
   Yahoo! Inc.

   EMail: addison@inter-locale.com


   Mark Davis (Editor)
   Google

   EMail: mark.davis@macchiato.com or mark.davis@google.com










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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
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Acknowledgement

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   Administrative Support Activity (IASA).







Phillips & Davis         Best Current Practice                 [Page 20]


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