Cantus firmus

[2] The earliest polyphonic compositions almost always involved a cantus firmus, typically a Gregorian chant, although by convention the term is not applied to music written before the 14th century.

Later, the cantus firmus appeared in the tenor voice (from the Latin verb 'tenere', to hold), singing notes of longer duration, around which more florid lines, instrumental and/or vocal, were composed or improvised.

In the 14th century, the technique continued to be widely used for most sacred vocal music, although considerable elaboration began to appear: while most continental composers used isorhythmic methods, in England other composers experimented with a "migrant" cantus firmus, in which the tune moved from voice to voice, but without itself being elaborated significantly.

The cantus firmus was at first restricted to the tenor, but by the end of the century many composers experimented with other ways of using it, such as introducing it into each voice as a contrapuntal subject, or using it with a variety of rhythms.

[5] There are numerous other examples of secular cantus firmi used for composition of masses; some of the most famous include: "Se la face ay pale" (Dufay), "Fortuna desperata" (attributed to Antoine Busnois), "Fors seulement" (Johannes Ockeghem), "Mille Regretz", and "Westron Wynde" (anonymous).

In the opening movement of Bach's St Matthew Passion, the chorale "O Lamm Gottes, unschuldig" appears in long notes, sung by a separate choir of boys "in ripieno".