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.

8. Their names are therefore due to numerical value; for when the voice becomes stationary on some one note, and then, shifting its pitch, changes its position and passes to the limit of the fourth note from that one, we use the term “fourth”; when it passes to the fifth, the term is “fifth.” [*](The remainderof this section is omitted from the translation as being an obvious interpolation)