Organum

The Organum was a medieval musical practice in which a plainchant melody was sung with at least one added voice. In its early stages, an organum involved only two voices: the principalis, which was notated, and the organalis, which was a usually improvised part, sung at a parallel consonant interval. Eventually. composers started writing out these parts, adding more lines, freeing the voices from the exclusively parallel movement and, in short, setting the bases of modern counterpoint.