Cantus firmus
A pre-existing melody as the basis of a composition.
What it means
A cantus firmus — “fixed song” — is a pre-existing melody that serves as the basis of a polyphonic composition. The other voices are invented around it. The cantus firmus itself often comes from another context: from liturgical chant, from a church hymn, or from a secular song.
Musical use
From the Middle Ages into the sixteenth century the procedure was a foundation of polyphonic music. In early polyphony the given melody usually lay in the lower voice and moved in long note values while one or more voices above it moved more quickly. The voice that carried the cantus firmus was called tenor, from the Latin word for holding. In the Mass composition of the fifteenth and sixteenth centuries, a cantus firmus was often carried through all the movements of a Mass and bound them into a whole; sometimes it came from a secular song after which the Mass was then named.
In Protestant church music the procedure lived on in the chorale setting: a chorale melody lies in one voice, often in long values, and is surrounded by contrapuntal voices.
In the study of composition the cantus firmus is also a teaching tool. Species counterpoint gives a short melody in equally long values against which the student writes counterpoints in five successive species.
The cantus firmus is to be distinguished from the fugue subject: the latter is taken up and worked out by all the voices in turn, whereas the former remains in one voice and is not broken up.
In the fifteenth and sixteenth centuries entire Masses were composed on a cantus firmus that returns in all movements, usually in the tenor and in long note values. Chorale melodies served as models as well as secular songs.
Performance practice
If you carry the cantus firmus, keep the long values calm and sustaining without letting the moving voices pull you along. Lead the line over its whole length instead of starting each note individually. And if you sing a counter-voice, listen to the cantus firmus as your point of reference — your dissonances and resolutions are governed by it.