DIAPHONY. — A type of composition in early polyphony (see ARS ANTIQUA), whose name is used by medieval theorists (9th–12th centuries) as equivalent to *organum*, indicating the technique in which parallel motion still predominates, whereas in later periods contrary motion will prevail.
Rules for diaphony are found especially in chapter 13 of the *Musica enchiriadis* of the 10th century (see Gerbert, *Scriptores ecclesiastici de musica sacra potissimum*, St-Blasien 1784, I, pp. 165–66) and in chapters 18 and 19 of Guido d’Arezzo’s *Micrologus* (ibid., II, chaps. 18, 19, pp. 21–23). In the first treatise, the prescribed intervals are the 4th, 5th, and octave, while in the second, major and minor 2nds, 3rds, and 4ths are proposed.