Cantus Firmus, in literal translation “firm chant” is a fixed melody taken from plainsong (which later became known as Gregorian chant), that composers of the 14th through 17th century used as the basis of polyphonic composition, against which other tunes are set in counterpoint. During the 16th century, cantus firmi (pl.) were used as the upper voice of a choir (see WFTD: Lied). In Joseph Fux’s pedagogical work of music theory, Gradus ad Parnassum, cantus firmus formed the basis of teaching his species counterpoint.

The cantus firmus is traditionally written in alto clef, beginning and ending on the tonic of the key. The note durations are of equal value, with the whole note being the traditional value. The range of the cantus firmus melody is usually no more than an octave. Occasionally the range stretches as far as a 10th, but most cantus firmi move within a much smaller range; limited to a 6th or even just a 5th above the tonic. German composers in the Baroque period, most notably Bach, used chorale melodies as cantus firmi.