In music, a cantus firmus is a pre-existing melody forming the basis of a polyphonic composition, often set apart by being played in long notes.
Composition using a cantus firmus was a common technique in Medieval music, forming the basis of organum as well as 13th- and 14th-century motets. In these works the cantus firmus was originally always taken from Gregorian Chant and was the fixed melodic material, moving in long notes, around which other more florid lines, instrumental and/or vocal, were composed. (This line was usually allocated to the tenor, from the Latin verb 'tenere', to hold).
In the early Renaissance composers experimented with other ways of using the cantus firmus, such as introducing it into each voice as a contrapuntal subject (theme), or using it with a variety of rhythms, or using secular tunes for canti firmi, even in sacred compositions.