The libretto may be serious or comic, although neither form necessarily excludes elements of the other. Opera differs from operetta in its musical complexity and usually in its subject matter. It differs also from oratorio, which is customarily based on a religious subject and is performed without scenery, costumes, or stage action. Although both opera and operetta may have spoken dialogue, in opera the dialogue usually has musical accompaniment, such as the harpsichord continuo in the operas of Mozart and Rossini.
Often, the music in opera is continuous, with set pieces such as solos, duets, trios, quartets, etc., and choral pieces, all designed to dramatize the action and display the particular vocal skills of the principal singers. For example, the last act trio from Gounod's Faust gives Mephistopheles (bass), Faust (tenor), and Marguerite (soprano) excellent opportunity to display their vocal talents singly and then weave their voices in ensemble singing as the two men vie for the soul of Marguerite, who is intent on salvation.
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