De institutione musica IV.13: De stantibus vocibus ac mobilibus.
Of all these voices some are wholly fixed, some wholly movable, and some sound neither wholly fixed nor wholly movable. Wholly fixed are proslambanomenos, hypate hypaton, hypate meson, mese, nete synemmenon, paramesos, nete diezeugmenon, nete hyperboleon, because in all three kinds they are the same, changing neither places nor names, whether they contain pentachords or tetrachords; pentachords, as proslambanomenos to hypate meson and mese to nete diezeugmenon; tetrachords, as hypate hypaton to hypate meson and hypate meson to mese. Movable are those which are changed according to each kind in this way: the paranetai and lichanoi of the diatonic and chromatic, the tritai and parhypatai of the enharmonic. For paranete hyperboleon diatonos is one thing, paranete hyperboleon chromatica another, trite enarmonios another. Paranete diezeugmenon diatonos and chromatica are also different, and are not the same as the trite diezeugmenon enarmonios in the other kinds. Nor are paranete synemmenon diatonos and chromatica and trite synemmenon enarmonios the same as the tritai in the remaining kinds. Lichanos meson diatonos and lichanos meson chromatice also differ, and parhypate meson enarmonios is found like the parhypate of no other kind. Nor do lichanos hypaton diatonos and lichanos hypaton chromatice keep the same places and numbers. For parhypate hypaton enarmonios is found unlike the parhypatai of the other kinds. Not wholly fixed or movable are those which remain in two kinds, the chromatic and the diatonic, but are changed in the enharmonic. This is considered thus. Trite hyperboleon diatonos and trite hyperboleon chromatice are described in the figure above by the same numbers, 2,916. But when we look at the enharmonic kind, we find another trite, that is 2,994. The voice, therefore, which was common to two kinds, is changed in the third. It is the same in the diezeugmenon tetrachord. For trite diezeugmenon diatonos and trite diezeugmenon chromatica are the same and agree with each other, but trite diezeugmenon enarmonios is distant from the former. In the synemmenon it is the same. For trite synemmenon diatonos and trite synemmenon chromatice are the same, but trite synemmenon enarmonios is different. Likewise parhypate meson diatonos and parhypate meson chromatica are marked the same, but in the enharmonic kind, as above the trite, so here the parhypatai are found next to the hypatai meson, but differ from the rest in force and pitch of sound. Again, parhypate hypaton diatonos and parhypate hypaton chromatica are the same, but not the same when sought in the enharmonic kind. But that the incomplete changeability of these may appear more clearly, let us return to the hyperboleon tetrachord. In this, then, the trite hyperboleon which is in the diatonic and chromatic kind is changed in the enharmonic and becomes paranete. Likewise the trite diezeugmenon which was called so in the diatonic or chromatic kind is called paranete in the enharmonic. The trite synemmenon which was in the chromatic or diatonic passes in the enharmonic into paranete. The parhypate meson which was seen in the chromatic or diatonic is found as lichanos meson in the enharmonic; and the parhypate hypaton which was called so in the diatonic or chromatic is named lichanos hypaton in the enharmonic. Fixed, then, are proslambanomenos, hypate hypaton, hypate meson, mese, nete synemmenon, paramesos, nete diezeugmenon, nete hyperboleon; movable, those which we call lichanoi or paranetai, whether diatonic or chromatic or enharmonic; not wholly movable or fixed, those which we call parhypatai and tritai in the diatonic or chromatic, and lichanoi or paranetai in the enharmonic kind.