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During the earlier medieval period, the vocal music from the liturgical genre, predominantly Gregorian chant, was monophonic, using a single, unaccompanied vocal melody line. Polyphonic vocal genres, which used multiple independent vocal melodies, began to develop during the high medieval era, becoming prevalent by the later 13th and early 14th century.

When did the Polyphonic vocal genre begin to develop?
during the high medieval era