DU@ according to art-is-jokken
In music of the Renaissance and early Baroque eras, a bicinium (pl. bicinia) was a composition for only two parts, especially one with a pedagogical purpose. Students could be expected to master singing a single part in a duet more easily than a part in a larger ensemble. Usually a bicinium was designed to be sung or played by students of the same age and ability, rather than for a single student and a teacher.
This model of moving from two-part study, writing, and singing to three parts and then more was adopted by Heinrich Glarean in his Dodecachordon (1547), one of the most influential music theory and pedagogy treatises of the Renaissance.
( source Wikipedia )