The [RFC 5646] coding scheme is constructed from a primary subtag component encoded using the shortest language codes of [ISO 639], plus codes for extensions for languages not represented in [ISO 639]. The code optionally includes additional subtag components, for scripts encoded using the four letter codes of [ISO 15924], and for regions encoded using the two letter country codes of [ISO 3166].

[RFC 5646] may be obtained at http://www.ietf.org/rfc/rfc5646.txt. [RFC 5646] obsoletes [RFC 4646], [RFC 3066] and [RFC 1766], but is forward compatible with those specifications. In previous editions of the Standard, [RFC 4646] codes were used with a Coding Scheme Designator of IETF4646. [RFC 5646] is a superset of [RFC 4646], which formalizes support for [ISO 639-3].

[ISO 639] codes may be obtained at http://www.loc.gov/standards/iso639-2/langhome.html.

The two letter country codes of [ISO 3166] may be obtained at http://www.iso.org/obp/ui/#search/code/

IANA language tag registrations may be obtained at http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

In previous editions of the Standard, this Context Group formerly included the three letter language codes of [ISO 639-2]/B, using Coding Scheme Designator ISO639_2, or the language codes of [RFC 3066], using Coding Scheme Designator RFC3066, and several IANA-registered language code extensions, using Coding Scheme Designator IANARFC1766.

In previous editions of the Standard, this Context Group provided only language identifiers, with national or regional variant identified in a separate Attribute or Content Item.