Ever since George Allen & Unwin decided to publish The Lord of the Rings in three volumes, the trilogy has ruled the fantasy genre to the point of becoming a cliché. Admittedly fantasy didn’t invent the trilogy–it goes back at least as far as the Victorian three-volume novel–but it certainly picked it up and ran with it. So why has the form persisted for so long?

Part of it is the Western obsession with the number three, particularly in relation to story structure. From The Three Little Pigs via the three caskets in Shakespeare’s Merchant of Venice to the structure of many jokes (An Englishman, an Irishman and a Scotsman walk into a bar…), the three-step story forms a pattern we all know and take for granted. Setup. Complication. Resolution. The three-act structure of Aristotle, now enshrined in scriptwriting dogma.

Is it something the publishers are doing? After all, we read so much about author X getting a three-book deal. Well, yes, publishers like to pin a writer down with multi-book deals, because a lot of work goes into establishing that author’s brand through jacket design and so on, and if the first book turns out to be a massive hit they don’t want their star to be lured away by another publisher with deeper pockets. But multi-book deals are the norm in most genres, regardless of whether the books themselves are standalones (as is usually the case in mainstream, romance, etc) or part of an open-ended series (a favourite in crime/mystery, ever since the days of Sherlock Holmes).

So if it’s not the publishers per se, who is driving the desire for trilogies? My feeling is that it’s the readers. Publishers want to buy what sells, because they’re in business to make money – and what sells most reliably is trilogies. Or tetralogies. Short, finite series.

Part of it is setting. A great fantasy story depends, more than anything else, on great world-building. And building a whole world takes a lot of the writer’s investment of time. Why throw all that hard work away and start again from scratch? And yet… Terry Pratchett has built his career on a single setting, the Discworld, without ever writing what could be called a trilogy in the usual sense. Every Discworld book is pretty much a standalone, even though it may be a sequel to one earlier in the series or at least feature some of the same characters.

However Pratchett’s books are small-scale, focusing on individuals and usually satirising one specific topic, be that football, the early days of cinema, or whatever. The true home of the trilogy is epic fantasy. And perhaps now we approach the crux of the matter. Epic fantasy is, thanks to Tolkien and his imitators, the most high-profile sub-genre of fantasy. Generally set in a secondary (i.e. invented) world, it uses a broad canvas to tell sweeping tales of adventure involving multiple cultures and/or sentient species. Even for a writer who doesn’t waffle or get sidetracked by his huge cast of characters, it takes a lot of words to describe such a story. Usually at least 250,000 of them and often a lot more. And it’s at this point that publishers balk at the page count and want to publish the story in three volumes.

The trilogy has become such a staple of the genre that it’s the go-to structure even for writers like me who prefer each book to have a self-contained plot. I have a manuscript on submission and have been explicitly asked “how a second and possibly a third book in this series might pan out”, so I have to at least consider making it a trilogy. I have no problem with that – the novel deliberately leaves some plot threads open because the background conflict is way too big to resolve in one book – but I do cringe ever so slightly at the thought of “Book One of (insert pretentious series title here)” appearing on the cover.

So, how do you as a fantasy reader, or writer, feel about trilogies? Love ‘em? hate ‘em? Classic or cliché?