Term, originally connected with the organ but later with ‘consonant music’, used for medieval polyphony; from the 12th century specifically referred to music with a sustained-note tenor (usually based on a pre-existing melody) and faster-moving upper part(s). From the late 13th century it was used for plainchant in general, as distinct from the motet and conductus.
The concept of consonance and the idea of a voice (vox organalis) added to a pre-existing chant melody (vox principalis) were fundamental to the development of organum. In the early Middle Ages the vox organalis generally lay below the vox principalis and the concept of consonance was based on parallel 4ths and 5ths. Influenced by the theoretical need to avoid the tritone and the demands of the tetrachord system, other intervals became acceptable; by the 11th century parallel movement in 3rds and contrary motion were important and the vox organalis enjoyed greater freedom of movement, eventually becoming established as the upper voice.
As the century progressed, a distinction was made between organum, in which the vox organalis forms melismas over the sustained notes of the vox principalis, and discant, where it forms more or less strict note-for-note counterpoint. In the melismatic type the vox organalis became the ‘main voice’ in effect, and drew on a number of melodic formulae (see Centonization) in relatively short melismas. The extension of these melismas marks the achievement of the Notre Dame school (notably Léonin and Pérotin) as represented in the Magnus liber (c 1170). This collection also reveals that, towards the end of the 12th century, melismatic organum was gradually superseded by the more fashionable discant counterpoint introducing modal rhythm. Non-modal melismatic organum (organum purum) disappeared, but a third type emerged in which modal rhythm was cultivated in the melismatic upper voice over the sustained notes of the tenor (as the vox principalis became known in the 13th century). Modal rhythm opened up new possibilities for three- and four-voice music, and was eventually responsible for the transformation to mensural, or rhythmically measured, polyphony.
