Rondo
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The rondo is a musical form that contains a principal theme which alternates with one or more contrasting themes, generally called "episodes", but also occasionally referred to as "digressions" or "couplets". Some possible patterns include: ABACA, ABACAB, ABACBA, or ABACABA.
Title page of Franz Rigler's "Three Rondos" (1790)
Aria
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In music, an aria is a self-contained piece for one voice, with or without instrumental or orchestral accompaniment, normally part of a larger work.
The farewell aria of Sultan Bazajet in Handel's opera Tamerlano (note the da capo instruction). First edition, London, 1719.
Gluck in a 1775 portrait by Joseph Duplessis