What is Diaphony?
(1) Dissonance as opposed to symphonia or consonance. ‘Dissonantia et diaphonia idem sunt: nam, ut dicit Isidorus, diaphoniae sunt voces discrepantes sive dissonae, in quibus non est jocundus sed asperus sonus.’
(2) A primitive form of discant, also known by the name of organum, described by Hucbald and Guido, in which the melody of the vox principalis was accompanied by the vox organalis at the fourth below, subject only to certain rules for the avoidance of the dissonant tritone:
Hucbald’s scale was laid out in tetrachords:
The four notes of the second tetrachord being the four finals of the church modes.
The golden rule of diaphony for the avoidance of the triton, or dissonance between the second note of one tetrachord and the third note of the tetrachord below, is that the vox organalis must never descend below the fourth note of a tetrachord, though it may move from one tetrachord to another according to the movement of the vox principalis.
Hucbald illustrates this by examples of the same melody in each of the four authentic modes:
This transposition is not apt for organal response because the B is generally sung natural.
In the first of these examples Gerbert prints B flat and A as the last two notes of the organum.
Hucbald assures us that diaphony, if sung slowly and with the gravity which befits it – ‘quod suum est hujus meli,’ cannot fail to produce a pleasing effect.
Guido gives us the additional information that the intervals of a whole tone, major third and perfect fourth are admissible, but not that of a semitone, and rarely that of a minor third.
It follows that the fifth, sixth, and seventh modes, which have a whole tone, major third and perfect fourth above F, C, and G respectively, are best adapted for diaphony, and that the third and fourth modes, which have a semitone and minor third above B sharp and E respectively, are the least suitable.
The vox organalis should not as a rule descend below C or in the higher registers below F. If a B sharp occur in the vox principalis, the organum must take G.
The close may be either at the fourth below or at the unison. In the latter case the box organalis should rise to the unison by a whole tone or major third, not by a semitone or minor third.
The term diaphonia is also applied by John Cotton and other writers to the next stage in the development of discant, in which contrary motion is employed, and the vox organalis moves freely both above and below the canto fermo.
The following example is from the anonymous treatise printed at p. 225 of Coussemaker’s Histoire de l’Harnomie au Moyen Age.
By the 13th century the term diaphonia for polyphony had generally given place to discantus, though even so late a writer as Johannes de Muris speaks of ‘diaphonia sive discantus’.