In the high medieval period, Europe began to move away from the simple monophony that characterized most world music. The first development in this direction was a style known as organum.

In the simplest form of organum, a melody is sung identically, but an octave apart to enrich the sound. This is arguably still unison singing. In parallel organum, however, melodies are sung separated by a certain fixed interval, usually a perfect fifth.

The next development took one of these voices and held it on a single pitch, creating a drone organum. Although this makes the second part simpler, drone organum is more sophisticated from a theoretical perspective, because it introduces a variety of different intervals into the music. This was the first foray into harmony, the distinguishing element of European music.

Organum laid the groundwork for polyphony, where multiple melodic lines move independently of each other. Organum with two melodic lines is called organum duplum; three lines make organum triplum; four lines create organum quadruplum.

The rules for composing organum are laid out in a ninth-century work entitled the Musica enchiriadis, which uses a rare notation system called Daseian Notation.