A trilogy is a series consisting of three distinct works that share a common thread. Literature, film, and the visual arts are popular mediums for this format. Many artists and writers use trilogies to tell an intricate and often intense story, as the three sections of a literary trilogy often allow for a greater amount of detail and back-story than would be possible with a single-story narrative form. The first known trilogies date back to ancient Greek theater, and modern forms of this narrative structure usually need to meet certain criteria to be classified as true trilogies.
Fine artists who create paintings or sculptures may do so in a series of three works that are sometimes labeled a trilogy, although this classification is technically inaccurate. Three pieces of art are generally designated a triptych when they can be included in a larger body of the same artist's work. A piece of classical music with three distinct sections is normally known as a sonata rather than a trilogy. The Dionysian theater festivals of ancient Greece were the setting of plays performed as trilogies, although some of them actually had four rather than three distinct sections.
Literature is one of the most common mediums for trilogies that follow the adventures of the same characters throughout all three stories. Popular trilogy authors often end one story with a suspenseful situation, so the readers need to move on to the next story in order to learn how it gets resolved. Fantasy and science fiction stories are usually favorite genres for this three-part narrative structure. The exact plot and sequence of a three-book trilogy can follow one of a few different formats depending on different writers' choices.
The first book of one of these trilogies usually introduces the characters, setting, and problems to be resolved. A second book serves as a sequel that often continues the story of the first one. Third installments of trilogies can offer a final resolution where all of the readers' questions are answered and the full narrative is complete.
Some authors choose to create a prequel out of the third book. In this case, the story cycles backward to a time before the action of the first book ever takes place. The purpose of this structure is to fill in a back-story that gives the reader a greater understanding of the circumstances that initially set the entire narrative in motion.