In the early treatises, the diverse techniques of improvisation are often presented as various « ways » of making up one or several counterpoints above (or around) a cantus firmus.
The improvised quality of these practices is not always mentioned ; it can often be inferred from the very simplicity of the indications that the improvisers are provided with.
We draw these melodies —which we make the cantus firmus of our improvisations— from song-books, graduals and antiphonaries.
Singing upon the book
Singing upon the book consists of taking a melody written in a book and adding to it extempore —without notation— one or several extra lines, according to strict contrapuntal rules. Polyphony is thus created by the superimposition of improvised voices —2 to 5 parts— in keeping with the style selected.