Most trilogies are works of fiction involving the same characters or setting, such as The Deptford Trilogy of novels by Robertson Davies or The Godfather films of Francis Ford Coppola. Others are connected only by theme: for example, each film of Krzysztof Kieślowski's Three Colors trilogy explores one of the political ideals of the French Republic (liberty, equality, fraternity) and each novel in Paul Auster's The New York Trilogy uses formats from detective fiction to explore existential questions. Trilogies can also be connected in less obvious ways, such as "The Nova Trilogy" of novels by William S. Burroughs, each written using Brion Gysin’s cut-up technique.
Occasionally, the term is applied to music, such as the Berlin Trilogy of David Bowie, linked together by their musical sound and lyrical themes, and the fact that part of them was recorded in Berlin, Germany.
A trilogy is different than a triptych, which is three related or connected paintings that are created at one time and designed to be viewed as a single work.
In the Dionysia festivals of ancient Greece, for example, trilogies of plays were performed followed by a fourth satyr play. The Oresteia is the only surviving trilogy of these ancient Greek plays, originally performed at the festival in Athens in 458 BC. The three Theban plays, or Oedipus cycle, by Sophocles, originating in 5th century BC, is not a true example of a trilogy because the plays were written at separate times and with different themes/purposes.
In ancient India, an example of an early trilogy includes the epic Mahabharata, which originally consisted of three portions. Vyasa's original core portion of the epic was the Jaya. Vaisampayana's Bharata expanded on the story, with Vyasa's Jaya embedded within it. Ugrasrava eventually composed the Mahabharata, with both Vyasa's Jaya and Vaisampayana's Bharata embedded within the epic.
Richard Wagner's epic series of operas, Der Ring des Nibelungen, is sometimes referred to as a trilogy even though it consists of four works: Das Rheingold, Die Walküre, Siegfried, and Götterdämmerung. The first work, Das Rheingold, is more correctly considered a prelude despite the fact that it is longer than most operas. After this prelude, which sets up the story, the trilogy begins with Die Walküre. Performances of The Ring are usually billed as three nights plus a prelude.
By contrast, The Foundation Series by Isaac Asimov originally consisted of Foundation, Foundation and Empire and Second Foundation and was considered a trilogy. Asimov wrote several more Foundation books and retroactively incorporated many of his other works into the continuity of the series. Despite this, the first three books are still considered a trilogy because they contain a story that is self-contained. Further complicating the matter, the Foundation series was originally eight short stories and novelettes written for science fiction magazines; its division into three books is more or less incidental.
The first three novels in the Hitchhiker's Guide to the Galaxy series were dubbed a trilogy, and even after he extended the series to five novels, author Douglas Adams, for humorous effect, continued to dub it a trilogy. He even called Mostly Harmless "the fifth book in the increasingly inaccurately named trilogy."
The six Star Wars films are generally separated into two trilogies; the "original trilogy" (the three films released between 1977 and 1983) and the "prequel trilogy" (the three films released between 1999 and 2005, which take place before the original three films).
Three works with similar themes from a creator may later come to be known as a trilogy, especially if produced one after the other. The Steven Spielberg films A.I.: Artificial Intelligence, Minority Report, and Catch Me If You Can are unofficially known as "the running man trilogy," because each featured a main character escaping a pursuer. Terry Gilliam has dubbed his films Time Bandits, Brazil and The Adventures of Baron Munchausen as "The Imagination Trilogy", in that each movie has to do with the imagination of humans in the three stages of life; child, man, elder man. Another example is the Dollars Trilogy by Sergio Leone; no continuity between the three movies was intended by Leone, but American marketers advertised the Clint Eastwood character in each film as being the same "Man with No Name".
One of the most popular "trilogies" of fantasy books, The Lord of the Rings by J. R. R. Tolkien, is not a trilogy, though it is often referred to as such. Tolkien regarded it as a single work and divided it into a prologue, six books, and five appendices. Because of post-World War II paper shortages, it was originally published in three volumes. It is still most commonly sold as three volumes, but has also been published in one-volume and seven-volume editions (six books and the appendices).
Occasionally, more than three works are planned but never finished. The Gormenghast fantasy trilogy is a trilogy by default, as author Mervyn Peake planned to write more novels set in that fictional world until his health failed.
In contrast, some works that were originally intended to be trilogies have been reclassified due to subsequent additions. Christopher Paolini changed the name of his "The Inheritance Trilogy" to The Inheritance Cycle, having announced that he would be writing a fourth book. However, Star Wars is an example of how the three original works may be regarded as a trilogy and the works as separate.