Trilogy

A trilogy is a set of three distinct works that are connected and can be seen either as a single work or as three individual works. Most trilogies are fiction involving the same characters or setting; others are connected only by theme. Trilogies date to ancient Greece: connected plays performed at the Dionysia followed by a fourth satyr play, with the Oresteia (458 BC) the only surviving example. Tolkien's The Lord of the Rings, a single novel published for economic reasons as three, helped popularize the modern format.

The series relation runs both ways: creators may later add more works, and the original three may or may not keep the "trilogy" title, as when Douglas Adams called Mostly Harmless "the fifth book in the increasingly inaccurately named trilogy". That drift from three to more is the planning question of how to write a series: each part must stand alone, and the whole must build one arc.