[Illustration: A—————————B C—————————D | E] “Let these chords be perfectly equal, and equally stretched, so as to be in unison, i. e. so that there may be only one sound, though there are two strings. But it is requisite that they should be placed upon some oblong and polished rule. The ancients called this rule an harmonic rule, or also a monochord, by which instrument all consonances and dissonances, and likewise musical intervals, were tried. Let now one of these chords be bisected in E. Afterwards under the point E place what is vulgarly called the _tactus_, but which was denominated by the ancients, from its figure, a hemisphere. The tactus, therefore, being placed under E, press there the chord, so that one half of it only, as for instance ED, may be wholly struck and resound. Having therefore struck each of the chords at the same time, viz. the whole of AB, and the half ED, so that they may resound at one and the same time, you will hear the sweetest of all consonances, composed from the sound of the whole chord AB, and the sound of the half ED. This consonance the ancients called diapason, i. e. _through all_ [the chords], because in the musical instruments of the ancients, the two extreme chords, i. e. the most grave, and the most acute of all the chords, contained this consonance; so that, from the gravest chord having made a transition through all the chords to the supreme and most acute of all, they would hear this sweetest consonance. It was, likewise, said to be in a duple ratio of the proportion of one sound to the other. For the sound of the chord AB is doubly greater or more grave than the sound of the half ED. For as sounding bodies are to each other, so are their sounds. But the chord AB is the double of ED. This, however, is now commonly called the octave, because from the first sound, and that the gravest, which is called _ut_, as far as to that sound which corresponds to it in the consonance diapason, there are these eight sounds, _ut_, _re_, _mi_, _fa_, _sol_, _re_, _mi_, _fa_. And of these the first ut, and the last _fa_, which is the eighth, produce the consonance diapason, or the double, or the octave. “Again, let the same chord CD be divided into three equal parts in the points F, G. [Illustration: A————————————B C————————————D | | F G]