Organum
Early polyphony in which further voices are added to a chant melody.
What it means
Organum is the earliest form of Western polyphony. To a Gregorian chant melody, the principal voice (vox principalis), a second voice is added, at first led in parallel at the distance of a fourth or fifth. The earliest descriptions are found in the treatise Musica enchiriadis around 900.
Musical use
Later the organum became freer: the added voice also moved in contrary motion, and the principal voice was led in long held notes over which the upper voice unfolded richly ornamented melodies. In the 12th and 13th centuries the Notre-Dame school in Paris reached a high point with Leoninus and Perotinus, with two-, three- and four-part settings and strict rhythm.
Organum should be distinguished from the later motet and from Renaissance counterpoint, which rest on equal, mostly texted voices, and from discant, a way of writing in which voices are led note against note.
Performance practice
When you sing early organum, pay attention to pure fourths and fifths and to the calm flow of the principal voice. The voices should listen to one another and blend in sound without covering one another.