De institutione musica I.21: De generibus cantilenae.
With these matters settled, we must speak of the kinds of melodies. There are three: diatonic, chromatic, enharmonic.
The diatonic is somewhat harder and more natural; the chromatic, already departing as it were from that natural tension and falling into softness; the enharmonic is the best and most aptly joined. Since there are, then, five tetrachords—hypaton, meson, synemmenon, diezeugmenon, hyperboleon—in all of these the song proceeds, according to the diatonic, by semitone, tone and tone in one tetrachord, then in another by semitone, tone and tone, and so on; and therefore it is called diatonic, as if it progressed by tone and by tone. The chromatic, however, which means colour, being as it were the first change from this tension, is sung by semitone, semitone and three semitones. For the whole diatessaron consonance is of two tones and a semitone, but not a full one. This word was drawn, so as to be called chroma, from surfaces, which, when they are changed, pass into another colour. The enharmonic, which is the more closely fitted, is sung in all the tetrachords by diesis and diesis and a ditone—a diesis being half of a semitone—so that the description of the three kinds, running through all the tetrachords, is in this manner: