B and T Codes
While most languages are given one code by the standard, twenty of the languages described have two three-letter codes, a "bibliographic" code (ISO 639-2/B), which is derived from the English name for the language and was a necessary legacy feature, and a "terminological" code (ISO 639-2/T), which is derived from the native name for the language. Each of these twenty languages is also included in the ISO 639-1 standard. (There were 22 B codes; scc and scr are now deprecated.)
In general the T codes are favored; ISO 639-3 uses ISO 639-2/T. However, ISO 15924 derives its codes when possible from ISO 639-2/B.
Read more about this topic: ISO 639-2
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“Thou hast a voice, great Mountain, to repeal
Large codes of fraud and woe; not understood
By all, but which the wise, and great, and good
Interpret, or make felt, or deeply feel.”
—Percy Bysshe Shelley (1792–1822)