dual: [17] Dual was borrowed from Latin duālis, a derivative of duo ‘two’ (which is a distant relative of English two). In Latin it was used particularly by grammarians, to denote the category ‘two people or things’ (as opposed to the plural, referring to three or more), and this was the earliest sense of the word adopted by English. (Incidentally, despite its formal similarity, and a common meaning element – two people participate – duel [15] is not etymologically related to dual; it comes from medieval Latin duellum, which was originally an archaic form of Latin bellum ‘war’. => two
