Cantus firmus, also known as a fixed melody or a pre-existing melody, is a pre-existing melody used as the basis for a polyphonic composition. It is a single line of music that serves as the foundation for a complex composition, often used in early music and church music. The cantus firmus may be placed in any voice, and the other voices are composed contrapuntally or harmonically in relation to it. The term "cantus firmus" comes from Latin, meaning "fixed song" or "firm melody."
