What is a cantus firmus?

A cantus firmus is a pre-existing melody used as the foundational basis for a polyphonic composition, particularly in the context of medieval and Renaissance music. It serves as a structural anchor around which composers create elaborate counterpoint, often in sacred music settings. Typically, the cantus firmus is presented in a slow, rhythmic manner, allowing other melodic lines to weave around it. This technique was widely employed in choral settings, especially in works like masses and motets.