De institutione musica I.23: Quae sint inter voces in singulis generibus proportiones.
In this manner, then, the division was made through the single tetrachords according to the properties of the kinds, so that all five tetrachords of the diatonic kind we divided by two tones and a semitone. And in this kind the tone is called uncompounded because it is set whole, and no other interval is joined to it, but in the single intervals there are whole tones. In the chromatic the division is set by a semitone and a semitone and an uncompounded trihemitone. We call this trihemitone uncompounded because it is placed in one interval. For in the diatonic kind a semitone and a tone can be called a trihemitone, but it is not uncompounded; for it is made up of two intervals. And in the enharmonic kind it is the same. For it consists of a diesis and a diesis and an uncompounded ditone, which we call uncompounded for the same reason, because it is placed in one interval.