Cantus firmus (fixed song) is the name given to a preexisting melody used as the basis of a polyphonic composition. The cantus firmus is set down first, and the other voices are adapted to it. This practice goes back to the earliest days of polyphonic writing, in which melodies familiar from sacred music or even popular tunes could provide the cantus firmus for a polyphonic work. The use of a cantus firmus is basic to the study of counterpoint, in which a student working though exercises like those in Fux's Gradus ad Parnassum will be given a cantus and required to write one or more additional lines to go with it.
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