Organum

Organum is early medieval polyphony that adds one or more melodic lines to a chant melody. Its changing styles trace the development of written and composed polyphony in Western Europe.

Parallel organum: A style of organum in which added voices move at a fixed interval alongside the chant. The earliest documented method doubles chant at a constant interval.
Musica enchiriadis: A late ninth-century musical treatise that gives the earliest surviving instructions for organum. It describes adding a second voice to chant in parallel motion.
Léonin: A composer associated with Notre-Dame whose music is described in the late thirteenth-century treatise Anonymous IV. Anonymous IV credits him with compiling a great book of organum.
Plainchant: The unaccompanied, monophonic liturgical singing of Western Christian traditions. Organum takes plainchant as its foundation and adds another voice.
Free organum: A style of organum in which added voices move more independently than in parallel organum. Its flexible voice-leading loosened the fixed intervals of earlier practice.
Scolica enchiriadis: A commentary on Musica enchiriadis that discusses early medieval music and organum. It expands the treatise’s account of how singers could perform organum.
Pérotin: A composer associated with Notre-Dame, known through descriptions in Anonymous IV. The treatise associates him with organum for three and four voices.
Conductus: A medieval Latin song, often polyphonic, whose melody was newly composed rather than borrowed from chant. Unlike organum, conductus generally does not build on a pre-existing chant melody.
Oblique motion: A musical motion in which one voice sustains or repeats a pitch while another moves. Organum uses sustained notes in one voice against movement in another.
Adémar of Chabannes: A French monk and historian whose writings include evidence about early polyphonic singing. His account offers an early witness to organum as a performed practice.

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