This part of ISO 639 provides a code consisting of language code elements comprising three-letter language identifiers for the representation of languages. The language identifiers according to this part of ISO 639 were devised for use in a wide range of applications, especially in computer systems, where there is potential need to support a large number of the languages that are known to have ever existed. Whereas ISO 639-1 and ISO 639-2 are intended to focus on the major languages of the world that are most frequently represented in the total body of the world's literature, this part of ISO 639 attempts to provide as complete an enumeration of languages as possible, including living, extinct, ancient and constructed languages, whether major or minor. As a result, this part of ISO 639 lists a very large number of lesser-known languages. Languages designed exclusively for machine use, such as computer-programming languages, an d reconstructed languages are not included in this code. Knowledge of the world's languages at any given time is never complete or perfect. Additional language identifiers may be created for this list when it becomes apparent that there is a linguistic variety that is deemed to be distinct from other languages in accordance with the definitions in clause 3 and their elaboration in clause 4. In addition, the denotation of existing identifiers may be revised or identifiers may become deprecated when it becom es apparent that they do not accurately reflect actual language distinctions. In all such changes, careful consideration is given to ensure existing implementations are not adversely affected.