trilogy

Dozens of tragic trilogies were written for the Greek stage, though only one, Aeschylus's great Oresteia (consisting of Agamemnon, The Libation Bearers, *and *The Eumenides), has survived complete. Authors in later years have occasionally chosen to create trilogies to allow themselves to develop a highly complex story or cover a long span of time. Tolkien's Lord of the Rings wasn't actually intended as a trilogy, but since it was published in three volumes it's usually called one. George Lucas's three original Star Wars movies are an example of a film trilogy (which he followed many years later by another).