De institutione musica V.15: Diatessaron consonantiam tetrachordo contineri.
Now we must speak of the division of the tetrachords.
For the diatessaron consonance is made up of four strings, and for this reason it is also called diatessaron. So that a tetrachord may be formed, with two strings placed at either side and sounding the diatessaron symphony, it is necessary to set two strings in the middle, which make three proportions to one another and to the extremes.