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
music, dance melodies, etc.
What began as the sequence of long notes now popular
ly 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 note
s early on in the study of counterpoint, which you use harmonically to write a contrapuntal melody to.