The cantus firmus is the basic melody on which a polyphonic piece is based.
Originally it could have been anything, from a chant, to a secular poem, or vulgar song and frequently ended being being the bottom voice. In cyclic masses, a single cantus firmus was used in each movement to give overall unity to the work.
With time various ways of adorning, devloping and altering the use of cantus firmus appeared, including mixing between voices, and interpolating other material, until, as a compositional tool , it ceased to be essential.
Typically the cantus firmus was written using a c clef, and this is, of course, suggested. This will also help keep an idea of vocal range in mind.
For these exercises you should use from eight to thirteen notes of the same value, just to get a feel for the melodic shape. This is the approach recommended in Gradus ad Parnassum, but ultimately when you write this music as your own composition, you will rarely be in such a situation.
There are a number of rules governing melody structure which can be learned here in a monophonic setting. Indeed, writing one's own cantus firmus is the best way to get a feel for the shape of the melodic style.
The rules are thus:
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