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Organum is a very early musical style of polyphony which involves a simple doubling of the chant at an interval of a fifth or fourth above or below. As organum evolved, the voices became increasingly independent, creating polyphony as we know it today.

The evolution of organum follows this scheme.

  • Parallel Organum at the Fourth or Fifth - 10th century
  • Oblique motion to Parallel Organum at the Fourth - 10th century
  • Use of Oblique, Contrary, and Parallel motion - 11 century
  • Florid Organum - 12th century

In Florid Organum a highly ornamented second voice was set against long notes in the "tenor". After the advent of Florid Organum, the older style of note against note was referred to as "discant" organum. Typical of composers using this technique were Leoninn and Perotin of Notre Dame in Paris.

In the 12th and 13th centuries, measured organum involved the alternation of cantus firmus organal and discant sections. In the organal section each note of the syllabic/neumatic section of the chant or cantus firmus was set against up to 40 notes in the second voice. In the discant section each note of the melismatic section of the cantus was set against one to three notes in the second voice.

Notre Dame Organum is most important in that rhythm was introduced according to a system of rhythmic modes. Leonin (late 12th c.) began to use the rhythmic modes in his 2-part works (organum duplum). Modal rhythm was even more prominent in the 3-part works (organum triplum) of Perotin. Polyphonic treatment was restricted to plainchants of the Graduals, Alleluias, Responsories, and the "Benedicamus Domino," but only the soloist sections were used for polyphonic treatment.