De institutione musica V.12: Quae sint aequisonae, quae consonae, quae emmelis.
Equisonants, then, are the diapason and the double octave, because from their blending and mixture one and, in a way, simple sound is produced. Consonants are, first, among the superparticulars, the sesquialter and sesquitertian, that is, the diapente and diatessaron; and the diapason with diapente and the diapason with diatessaron. These are composed and joined from equisonants and consonants. Emmelis are the remaining ones that can be placed among these, as the tone, the difference between the diatessaron and the diapente; and the equisonants are in a way joined to the consonants, as the diapason from diatessaron and diapente, while the consonants are joined from those sounds which are called emmelis, as the same diapente and diatessaron from tones and the other proportions to be spoken of later. But how the proportion of all these can be gathered must be taken from the place which we described at the end of the fourth volume, where the string was stretched over the half-spheres. For there are found the equisonance of the diapason and double octave, the simple consonances diapente and diatessaron, the composite consonances diapason with diapente and diapason with diatessaron, and those sounds which are emmelis, consisting in the difference of the tone.