Organum added what to Gregorian chants?

Organum introduced a form of early polyphony to Gregorian chants by adding one or more melodic lines above the original chant. This technique involved singing additional notes in parallel intervals, often a fourth or fifth above the chant melody, thereby enriching the texture of the music. Over time, organum evolved, leading to more complex harmonic structures and greater independence between the voices, which laid the groundwork for later developments in Western music.