Its principal characteristic was the rectilinear or parallel motion, in which the voices move while maintaining the same distance between them (the intervals admitted as consonant were the 4th, 5th, and octave). It began to decline in the 11th century, when contrary motion (in which the voices approach and recede in their progression together) was introduced into early polyphony, and the organum had to give way to the new form of discant (v.).
The technical structure of the organum consisted first of all of a pre-existing chant, taken from the liturgical repertoire, which was called the tenor because it sustained the Gregorian melody (usually chosen from Graduals, Responsories, and Alleluias). Other voices were then added above or below the first, at established and regular intervals. In an initial phase, the added voice (or vox organalis or duplum) was placed below the tenor (or vox principalis) at a distance of a 4th or a 5th. This form, the simplest, consisted of only two voices, was called parallel organum, and lasted throughout the first period in which rectilinear motion was generally practiced. In the 11th century, with the extension of that contrary motion which was already latent in the first phase, in which even the parallel organum began to end in unison, the free organum emerged, which was already practically a discant, in which the voices often crossed.
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 one of the tenor. This type of organum, called "purum" or "duplum," widely cultivated especially at St-Martial de Limoges, had as its principal characteristic a great rhythmic freedom in the added voice, while in the following period rhythmic modes influenced the school of Notre-Dame in Paris (Léonin and, even more, Pérotin) and led the organum toward a clear mensuralism. This was also necessitated by the increased number of added parts, which in Pérotin reached three in quadrupla organa.
The principal writers who discuss this form are: J. Scotus Eriugena, in the treatise *De divisione naturae* (c. 877); the author of the *Musica enchiriadis* (9th century); Regino of Prüm (915); Hucbald in *De harmonica institutione* (840-930); Guido d'Arezzo in the *Micrologus*; the author of the treatise *Ad organum faciendum* in the Ambrosian Library in Milan (11th-12th century); Johannes Cottonus in *De diaphonia* (11th-12th century); Elias Salomonis in his *Scientia artis musicae*, also in the Ambrosiana (13th century). These treatises also provide, as demonstration of the theories expounded, the first examples of organum (v. ARS ANTIQUA; DIAPHONIA).