Descant

Music that features a fixed vocal melody set to improvisations, often in counterpoint

A descant, discant, or discantus is any of several different things in music, depending on the period in question; etymologically, the word means a voice (cantus) above or removed from others. The Harvard Dictionary of Music states:Anglicized form of L. discantus and a variant of discant. Throughout the Middle Ages the term was used indiscriminately with other terms, such as descant. In the 17th century it took on special connotations in instrumental practice.

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