ORGANUM. — The term, which denoted a special compositional technique involving at least 2 voices, stands at the origins of polyphony (9th century) and soon came to signify an independent musical form.
Its principal characteristic was direct or parallel motion, in which the voices moved while always maintaining the same distance between them (the intervals admitted as consonances were the 4th, 5th, and octave), and it began to decline in the 11th century, when contrary motion was introduced into primitive polyphony (in which the voices draw closer together and move apart as they proceed together), and the o. had to yield its place discant (v.). The technical structure of the o. consisted first of all of a pre-existing chant, taken from the liturgical repertory, which was called tenor because it “held” the Gregorian melody (generally chosen from Graduals, Responsories, and Alleluias), and then of other voices added above or below the first, at established and regular intervals. In an initial phase, the added voice (“vox organalis” or “duplum”) was placed below the tenor (or “vox principalis”) at a distance of a 4th or 5th. This form, the simplest, consisted of only 2 voices and was called parallel o.; it lasted throughout the first period in which direct motion was generally practiced. In the 11th century, with the spread of the contrary motion that was already latent in the first phase—during which even the parallel o. began and ended in unison—there arose the free o., which is already practically a discant, in which the voices often come to cross one another. In the 12th century the “vox organalis” moved above the “vox principalis” and no longer followed it note against note, but developed broad melismas, that is, florid groups of several notes, against a single note of the tenor. This type of o., called “purum” or “duplum,” cultivated extensively especially at St-Martial de Limoges, had as its principal characteristic great rhythmic freedom in the added voice; in the following period, however, the rhythmic modes influenced the school of Notre-Dame in Paris (Leoninus and, even more so, Perotinus) and directed the o. toward clear mensuralism. This was also necessitated by the increased number of added parts, which in Perotinus reached 3 in the “organa quadrupla.” The principal writers who discuss this form are: G. Scoto Eriugena, in the treatise De divisione naturae (about 877); the author of Musica enchiriadis (9th century); Reginone di Prüm (915); Huckbald in De harmonica institutione (840–930); Guido d’Arezzo in Micrologus; the author of the treatise Ad organum faciendum in the Bibl. Ambrosiana of Milan (11th–12th century); Johannes, called Cotton, in De diaphonia (11th–12th century); Elias Salomonis in his Scientia artis musicae, also in the Ambrosiana (13th century). These treatises likewise provide, as demonstrations of the theories expounded, the earliest examples of o. (v. ARS ANTIQUA; DIAFONIA).