Cantus firmus

An existing melody that becomes the basis of a polyphonic composition through the addition of contrapuntal voices.  In terms of their origin, the cantus firmi can be divided into four groups: To group 1, which is by far the largest, belong all the organa and clausulae, practically all the motets of the 13th and 14th centuries, some motets and numerous Masses of the 15th and 16th centuries, the organ Magnificats, and the organ hymns of the 16th century.

The latter lead to group 2, which includes the organ chorales (chorale preludes) of Buxtehude, Pachelbel, Bach, Brahms, et al., as well as the chorale choruses in cantatas and Passions.

Group 3 includes some motets of the 13th and 14th centuries and many Masses of the 15th century.

The cantus firmus appears most frequently in the tenor, usually in long notes that contrast with the more florid design of the other parts.  In the 13th century, its rhythmic structure was determined by the system of the rhythmic modes, in the 14th by the principle of isorhythm.  In the 15th century and later the cantus firmus often occurred in long notes of equal value.