A trilogy is a set of three works of art that develop a single theme. Trilogies are commonly found in literature and occasionally in film (for example, Back to the Future). Some authors of popular trilogies may later extend the series by adding more works to it, thus technically making the series no longer a trilogy. This happened in the case of Isaac Asimov's Foundation series, which was originally three books, and also in The Hitch Hikers Guide to the Galaxy, a series of five novels which the author Douglas Adams, for humorous effect, continued to dub a "trilogy" by his own fiat for the rest of his life.
Other famous trilogies include:
- U.S.A., three novels by John dos Passos
- The Gormenghast trilogy by Mervyn Peake
- Three Colors movie trilogy by director Krzysztof Kieslowski
- The Star Wars movies now a double trilogy
- Indiana Jones movies although a fourth is rumored
- Peter Jackson's The Lord of the Rings movies
- Although it was only seen as such after their release, Parts 2-4 of the Star Trek feature film series is often considered a trilogy
- The Lord of the Rings book/books