The compound "two-party" is a modern English formation that transparently combines the numeral "two" with the noun "party." The word "two" descends from the Proto-Indo-European root dwóh₁, a fundamental numeral that gave rise to equivalents across many languages, signifying duality. "Party" originates from the Latin verb partire, meaning "to divide or share," which evolved through Old French to denote a faction or side taken in a division, eventually specializing to mean an organized political group. The logic of the compound is directly descriptive, specifying a system or situation characterized by the involvement of precisely two such political groups, reflecting the structural simplicity of its constituent morphemes.

Structure: two (number) + party (political group) - two: From Old English twā, of Germanic origin; related to Dutch twee and German zwei, from an Indo-European root shared by Latin duo and Greek dyo. Functions as a numerical determiner/modifier. - party: From Old French partie, based on Latin partire 'to divide'. The sense of a political group developed in the early 18th century. Functions as the head noun in the compound.