. A preexisting segment of music, usually extracted from Gregorian chant, upon which a new polyphonic composition is based. The use of a cantus firmus provided the earliest means of constructing polyphony and remained important as a compositional...

In music, a cantus firmus ("fixed song") is a pre-existing melody forming the basis of a polyphonic composition. The plural of this Latin term is cantus firmi, though one occasionally sees the corrupt form canti firmi. The Italian is often used instead:...