A Brief History of Western Music
Organum
When studying the historical development of music, it is extremely important to follow the development of polyphony. Polyphony, or polyphonic music, is when two or more melodies are played/sung simultaneously. This is the exact opposite of monophonic music, where only one melody is played/sung at any given time. An example of monophonic music is the Gregorian chant. In the early Middle Ages, music was almost universally monophonic. But, at some point, musicians began to
Pérotin, one of the two leading composers of the Notre Dame school.
experiment and place new melodies on top of existing Gregorian chants. This marked the birth of polyphonic music. Towards the end of the Middle Ages, musical compositions became increasingly complex, as composers added more and more lines of melody on top of old ones.
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Organum is the earliest form of polyphonic music. It consisted of a newly composed melody, written on top of an existing Gregorian chant. When used in organum, the old Gregorian chant was known as the cantus firmus (latin for "fixed song"). Organum was first recorded in the 9th century, but further developments were made in the 12th and 13th century.
The Notre Dame School
The Notre Dame school was a group of composers who lived in the 12th and 13th centuries. Named after the Notre Dame Cathedral, this school made major contributions to the development of polyphony. The two leading composers of the Notre Dame school were Léonin and Pérotin. Léonin was the first composer of polyphony known to us by name, and Pérotin was the first to compose three- and four-part organum (i.e. he composed music with three or four melodies played simultaneously).
Haec Dies (Organum)
On the Gregorian chant page, we discussed the chant Haec dies. In the 12th century, Haec dies was turned into an organum by an unknown composer. In the organum version of Haec dies, the cantus firmus (the original chant) is sung by a lower voice, while a newly composed melody is sung simultaneously in a higher voice. You can listen to a recording of it below: