The principal type of Italian Baroque opera was opera seria, or serious opera. The plots — mostly derived from ancient history, with all kinds of alterations and additions — were designed to stir up powerful emotions, such as passion, rage, grief, and triumph. Such plots gave the singers many opportunities to excel in one kind of expression or another. Opera seria consisted mainly of solo singing by sopranos and mezzo-sopranos, including castrati (see the box later in this chapter). Brilliant high voices were prized above all. Tenors and basses played subordinate roles, and there were few duets or choruses.

The words of an opera are called the libretto (“little book”), and their author is the librettist. Librettists had to build up the drama as a whole from a series of brief texts, alternating with one another, for recitatives and arias.