All existing parts of the series were consolidated into a single standard in 2023,[3] largely based on the text of ISO 639-4.
The language codes defined in the several sections of ISO 639 are used for bibliographic purposes and, in computing and internet environments, as a key element of locale data.
The codes also find use in various applications, such as Wikipedia URLs for its different language editions.
Its "language symbols" consisted of one- or two-letter variable-length identifiers in capitalized Latin alphabets, e.g. E or En for English; S, Sp, or Es for Spanish; and In for Indonesian.
Since then, the standard has been adopted as a fundamental technology of the rapidly expanding computer industry (RFC 1766), leading to development of more expressive three-letter framework, published as ISO 639-2:1998, largely based on MARC codes for languages.
Relatively constant updates in parts of ISO 639 had been handled by each own authority in charge until the publication of ISO 639:2023, which harmonized and reunified the body text of former standards and brought about organizational change with a joint maintenance agency supervising all sets and issuing newsletters.
These are groups containing multiple individual languages that have a good mutual understanding and are commonly mixed or confused.
Some macrolanguages developed a default standard form on one of their individual languages (e.g. Mandarin is implied by default for the Chinese macrolanguage, other individual languages may be still distinguished if needed but the specific code cmn for Mandarin is rarely used).
The common use of three-letter codes by three sets of ISO 639 requires some coordination within a larger system.