De institutione musica I.19: Diapason quinque tonis et duobus semitoniis iungi.
The diapason consonance consists of five tones and two semitones, which nevertheless do not fill up one tone. For since it has been shown that the diapason consists of diatessaron and diapente, and the diatessaron has been proved to consist of two tones and a semitone, the diapente of three tones and a semitone, joined together they make five tones. But since those two semitones were not exact halves, their union does not reach the full measure: it exceeds the half, but falls short of wholeness. And according to this account the diapason consists of five tones and two semitones, which, as they do not attain to a whole tone, so go beyond a whole semitone. But what the reason of these is, and how the musical consonances themselves are discovered, will be explained more clearly later. Meanwhile, in the present discussion, belief must be given under a moderate understanding; but then all firm faith must be taken up, when each point has become clear by its own demonstration. With these things thus arranged, let us discuss for a little the strings of the cithara and their names, how they were added, and what the cause of their names is. For when these have first come to our knowledge, it will be easy to embrace by knowledge what follows.