On Architecture

7. There are five tetrachords: first, the lowest, termed in Greek u(/paton; second, the middle, called me/son; third, the conjunct, termed fourth, the disjunct, named diezeugme/non; the fifth, which Greek u(perbo/laion. The concords, termed in Greek sumfwni/ai, of which human modulation will naturally admit, are six in number: the fourth, the fifth, the octave, the octave and fourth, the octave and fifth, and the double octave.