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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“We must trust infinitely to the beneficent necessity which shines through all laws. Human nature expresses itself in them as characteristically as in statues, or songs, or railroads, and an abstract of the codes of nations would be an abstract of the common conscience.”
—Ralph Waldo Emerson (1803–1882)