De institutione musica I.15: De ordine theorematum idest speculationum.
With these things thus laid down, it seems we must say in how many kinds all song is woven, which the discipline of harmonic invention considers. They are these: the diatonic, the chromatic, the enharmonic. These can be explained only if we first discuss the tetrachords and how the number of strings, which is now one of plurality, has increased to its present extent. This will be done if we first recall by what proportions the musical symphonies are mingled.