Preexistent melody, such as a plainchant (see Gregorian chant) excerpt, underlying a polyphonic musical composition (one consisting of several independent voices or parts). In the 11th- and 12th-century organum, the tones of the plainchant melody for such words as “alleluia” and “amen” were held by one voice (the tenor), while another, more active, improvised line was added. Developments introduced by the Notre-Dame school of the late 12th and early 13th centuries included rhythmic patterning of the added voice and the addition of two or three voices. The composition of nonliturgical words for the added voice or voices in the 13th century resulted in the independent motet. Cantus firmus technique remained the basis of most composition of the 14th–15th centuries (though the “chant” was now often a secular melody) and remained important in the 16th-century mass. It was later codified in the pedagogical method called species counterpoint.
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The plural of this Latin term is canti firmi, though one occasionally sees the corrupt form cantus firmi. The Italian is often used instead: canto fermo (and the plural in Italian is canti fermi).
Composition using a cantus firmus continued to be the norm through the 13th century: almost all of the music of the St. Martial and Notre Dame schools uses a cantus firmus, as well as most 13th century motets. Many of these motets were written in several languages, with the cantus firmus in the lowest voice; the lyrics of love poems might be sung in the vernacular above sacred Latin texts in the form of a trope, or the sacred text might be sung to a familiar secular melody.
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, however without itself being elaborated significantly. Elaborations came later, in what was to be known as the paraphrase technique; this compositional method became important in composition of masses by the late 15th century. (See paraphrase mass.)
The cyclic mass, which became the standard type of mass composition around the middle of the 15th century, used cantus firmus technique as its commonest organising principle. At first the cantus firmus was almost always drawn from plainchant, but the range of sources gradually widened to include other sacred sources and even sometimes popular songs. 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. During the 16th century the cantus firmus technique began to be abandoned, replaced with the parody (or imitation) technique, in which multiple voices of a pre-existing source were incorporated into a sacred composition such as a mass. Yet while composers in Italy, France, and the Low Countries used the parody and paraphrase techniques, composers in Spain, Portugal, and Germany continued to use the cantus firmus method in nationally idiosyncratic ways.
Probably the most widely set of the secular cantus firmus melodies was L'homme armé. Over 40 settings are known, including two by Josquin Desprez, and six by an anonymous composer or composers in Naples, which were intended as a cycle. Many composers of the middle and late Renaissance wrote at least one mass based on this melody, and the practice lasted into the seventeenth century, with a late setting by Carissimi. There are several theories regarding the meaning of the name: one suggests that the "armed man" represents St Michael the Archangel, while another suggests that it refers to the name of a popular tavern (Maison L'Homme Armé) near Dufay's rooms in Cambrai. Being that this music arose shortly after the Fall of Constantinople in 1453, it is possible that the text "the armed man should be feared" arose from the fear of the Ottoman Turks, who were expanding militarily towards central Europe. There are numerous other examples of secular cantus firmi used for composition of masses; some of the most famous include 'Fortuna Desperata' (attributed to Antoine Busnois), 'Fors seulement' (Johannes Ockeghem), 'Mille regretz' (Josquin), and 'The western wynde' (anonymous).
German composers in the Baroque period in Germany, notably Bach, used chorale melodies as cantus firmi. 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". Many of his chorale preludes include a chorale tune in the pedal part.