what is a cantus firmus?

A cantus firmus, also known as a fixed song or a pre-existing melody, is a musical term primarily used in Western classical music. It refers to a pre-existing melody that serves as the foundation for a composition, usually a polyphonic composition.

In the medieval and Renaissance eras, the cantus firmus was often a Gregorian chant or a popular secular tune. It provided a basis for composers to create intricate polyphonic textures by adding other voices or parts. The cantus firmus was typically written in a long note values, while the other voices would create counterpoint or complementary melodies around it.

During the Baroque period, the use of cantus firmus declined, but the concept continued to evolve. Composers would often incorporate well-known tunes into their works or use contrapuntal techniques to incorporate fragments of a melody into various parts.

Overall, the purpose of a cantus firmus is to provide a structural framework for a composition and to unify the different voices or parts in a polyphonic texture. The cantus firmus could be perceived as a musical anchor around which other melodies are crafted.