Organum

Organum is a technique of singing developed in the Middle Ages and is a primitive form of polyphonic music. In its earliest stages, organum involved two musical voices: a melody, and a second melody a transpositions of the first. The two voices are separated by a constant and consonant interval, usually a perfect fifth or fourth. Organum was originally improvised; while one singer performed a notated melody (the "vox principalis"), another singer--singing "by ear"--provided the unnotated second melody (the "vox organum"). Over time, composers began to write added parts that were not just simple transpositions, and thus true polyphony was born. Organum as a musical genre reached its peak in the late twelfth century with composers such as Perotin, who belongs to the so-called "Notre Dame school" of organum. See Medieval European Music.