A point that deserves special notice at this place is the use of the prefix Hypo- (hypo-) in the names of keys. In the final Aristoxenean system Hypo- implies that a key is lower by the interval of a Fourth than the key to whose name it is prefixed. This convention served to bring out the special relation between the two keys, viz. to show that they are related (to use modern language) as the keys of a tonic and dominant. In the scheme of keys now in question there is only one instance of this use of Hypo-, namely in the Hypo-phrygian, the most recently introduced. It must have been on the analogy of this name that the term Hypo-dorian was shifted from the key immediately below the Dorian to the new key a Fourth below it, and that the new term Hypo-lydian was given to the old Hypo-dorian in accordance with its similar relation to the Lydian. In the time of Aristoxenus, then, this technical sense of Hypo- had not yet been established, but was coming into use. It led naturally to the employment of Hyper- in the inverse sense, viz. to denote a key a Fourth higher (the key of the sub-dominant). By further steps, of which there is no record, the Greek musicians arrived at the idea of a key for every semitone in the octave; and thus was formed the system of thirteen keys, ascribed to Aristoxenus by later writers. (See the scheme at the end of this book, Table II.) Whether in fact it was entirely his work may be doubted. In any case he had formed a clear conception—the want of which he noted in his predecessors—of the principles on which a theoretically complete scheme of keys should be constructed.

In the discussions to which we have been referring, Aristoxenus invariably employs the word tonos in the sense of 'key.' The word harmonia in his writings is equivalent to 'Enharmonic genus' (genos enarmonion), the genus of music which made use of the Enharmonic diËsis or quarter-tone. Thus he never speaks, as Plato and Aristotle do, of the Dorian (or Phrygian or Lydian) harmonia, but only of the tonoi so named. There is indeed one passage in which certain octave scales are said by Aristoxenus to have been called harmoniai: but this, as will be shown, is a use which is to be otherwise explained (see p. 54).