De institutione musica I.25: Quid sit diazeuxis.
Diazeuxis is the name for what can be called disjunction, whenever two tetrachords are separated by the interval of a tone, as in these two tetrachords.
It is evident, then, that there are two tetrachords, since there are eight strings. But there is diazeuxis, that is, disjunction, between the mese and the paramese, which differ from each other by a full tone. Of these it will be explained more clearly when a later treatise takes up each point for more careful exposition. But to one who looks more carefully, five tetrachords, and no more, are found: hypaton, meson, synemmenon, diezeugmenon, hyperboleon.