The compound "two-letter" is a transparent Modern English formation, directly combining the numeral "two" with the noun "letter." The word "two" descends from a fundamental Proto-Indo-European root (duwo-) for the number, which is remarkably stable across Indo-European languages. "Letter" entered English from Latin littera, which referred to a written character and, by extension, an epistle or document. The logic of the compound is purely descriptive and compositional, denoting anything—be it a word, an initialism, or a symbolic code—that is comprised of exactly two alphabetic characters. Its formation follows the common English pattern of combining a number with a noun to create a precise classifier.

Structure: two (numeral) + letter (character) - two: From Old English 'twā', the feminine and neuter form of 'twēgen' (two). A numeral root of Proto-Germanic origin, ultimately from Proto-Indo-European duwo-. - letter: From Old English 'lǣþer', 'lēþer' (letter of the alphabet, written character), via Latin 'littera' (letter of the alphabet, in plural: epistle, literature). Its ultimate origin is uncertain but may be Etruscan.