Definitions for "Cantus firmus"
literally "firm chant," a pre-existing melody such as a chorale tune to which counterpoint has been set. In Italian.
(Lat.) : A melody from some other work borrowed to serve as the basis for a new polyphonic composition. Josquin's Déploration sur la mort de Joh. Ockeghem incorporates a plainchant Requiem in aeternam as cantus firmus (in the tenor voice). In the Renaissance, a great number of polyphonic masses were based on the famous song "L'Homme armé" (The Armed Man) as cantus firmus. Vestiges of the practice can be seen in the 19th-century fondness for weaving, for example, the Gregorian Dies irae into macabre compositions--as in the last movement of Berlioz's Symphonie fantastique.
("fixed melody") A pre-existing plainchant or secular melody incorporated into a polyphonic composition, common from the twelfth through the sixteenth centuries.
A singer in good physical condition. As opposed to the "Cantus phlabbious" (see Sackbutt).
Keywords:  get, notes, know, part, four
Part you get when you only know 4 notes.
The part you get when you can only play four notes.