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The Cantus Firmus is a fixed musical line against which counterpoint may be written.

These chants, usually the basis for polyphonic composition in the earliest stages of musical history, came from Gregorian chant, church chorales, vocal music, dance melodies, etc.

What began as the sequence of long notes now popularly employed in species counterpoint studies evolved through shorter note values and ornamentation; as such it is essential for the music student to work his way through counterpoint from simple (whole note against whole note first species counterpoint) to more complex (four part fugues) techniques. This process illustrates for the student music's evolution through history from antiquity to present day.

The Cantus should act as a fixed axis on which the creative impulse of the budding composer may fasten to and develop from.

To put it in the terms used in Baroque musical textbooks it may be said that "...the Cantus is the rule which corrects the emotion. When the student has become qualified to compose, the emotion will correct the rule."

In simpler terms, the Cantus Firmus is the bass line given, in whole notes early on in the study of counterpoint, which you use harmonically to write a contrapuntal melody to.