It's true: the following melody, a popular renaissance tune, was used as the cantus firmus in many masses, such as those by Guillaume Dufay, Johannes Ockeghem, and Giovanni da Palestrina.
A cantus firmus, or "fixed melody," in music is a pre-existing melody that serves as the foundation for a polyphonic work. Although the faulty form canti firmi (coming from the grammatically incorrect presentation of cantus as a second- rather than a fourth-declension noun) can also be found, the plural of this Latin term is cantus firmi. Canto Fermo is the often used Italian equivalent (and the plural in Italian is canti fermi).
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