In music, a cantus firmus ("fixed melody") is a pre-existing melody forming the basis of a polyphonic composition. The plural of this Latin term is , although the corrupt form canti firmi (resulting from the grammatically incorrect treatment of cantus as a second- rather than a fourth-declension noun) can also be found. The Italian is often used instead: (and the plural in Italian is ). History The term first appears in theoretical writings early in the 13th century (e.g., Boncampagno da Signa, Rhetorica novissima, 1235). 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. The earliest surviving polyphonic compositions, in the Musica enchiriadis (around 900 AD), contain the chant in the top voice, and the newly composed part underneath; however, this usage changed around 1100, after which the cantus firmus typically appeared in the lowest-sounding voice. 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. 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, but 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