Duple is from Latin 'duplus' ('twofold'), from 'duo' ('two') + '-plus' ('-fold'). The ancestry passes through Latin duplus and Latin duo. The suffix -fold marks multiplication or a repeated measure, so the English form is shaped by recognizable word formation as well as by its inherited or borrowed base. In present use as an adjective, it means "consisting of two parts, especially having two beats per measure", keeping the historical formation close to the word's ordinary semantic work. Related words include double, duplex, duplicate, and duo.