In music, cantus firmus is the basic material to be set using polyphony. The cantus firmus was originally always taken from Gregorian Chant and was the fixed melodic material, moving in whole notes, around which other more florid lines, instrumental and/or vocal, were composed around. (This line was usually allocated to the tenor, from the Latin verb 'tenere', to hold). Eventually the cantus firmus came to be set in a different rhythm in different movements of the same composition, or be made up or taken from a source besides chant (from popular music).

Probably the most widely set of the secular cantus firmus melodies was l'homme armé. Over 30 settings are known. Most early renaissance masters each set at least one mass on this melody, and the practice lasted into the seventeenth century, with a late setting by Carissimi. Some have suggested that the 'armed man' represents St Michael the Archangel, whilst others have suggested it merely represents the name of a popular tavern (Maison L'Homme Arme) near Dufay's rooms in Cambrai.

Other secular examples include 'Fortuna Desperata', 'Mille regretz' and 'The western wynde'.

Setting the cantus firmus was the essential pedagogical tool in Gradus Ad Parnassum by Johann Fux and formed the basis of teaching his species counterpoint.

Reference

Sparks, E. H. 'Cantus firmus in Mass and Motet', Berkeley, (1963)