Organum is, in general, a plainchant melody with at least one added voice to enhance the harmony, developed in the Middle Ages. Depending on the mode and form of the chant, a supporting bass line may be sung on the same text, the melody may be followed in parallel motion, or a combination of both of these techniques may be employed. As no real independent second voice exists, this is a form of heterophony. In its earliest stages, organum involved two musical voices: a Gregorian chant melody, and the same melody transposed by a consonant interval, usually a perfect fifth or fourth. In these cases the composition often began and ended on a unison, the added voice keeping to the initial tone until the first part has reached a fifth or fourth, from where both voices proceeded in parallel harmony, with the reverse process at the end. Organum was originally improvised; while one singer performed a notated melody, another singer—singing "by ear"—provided the unnotated second melody. Over time, composers began to write added parts that were not just simple transpositions, thus creating true polyphony.
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