De institutione musica V.19: Quemadmodum Ptolomaeus tetrachordorum divisionem fieri dicat oportere.
…Ptolemy divides the tetrachords on a different principle, laying down at the outset that between the two sounds at either end such little voices should be fitted as exceed one another in superparticular proportions, yet unequal ones, since a superparticular proportion cannot be divided into equal parts; next, that every comparison made to the string which is the lowest in the three should be smaller than the rest that are joined to the high voices. But in these, the genera we call dense must be such that the two proportions nearest the lowness are smaller than the proportion which is left toward the high. In the non-dense, however, as in the diatonic genera, never one.