Diptychs, dilogies, duologies, series, cycles, and sagas
TL;DR: Use diptych for one novel published in two halves, dilogy or duology for two completely different but still ordered novels, but just series when ordering doesn’t matter. Less commonly used words for related collections include cycle, saga, and legendarium.
Actually, a series of three may or may not be a trilogy — it just depends. It might just be a three-part serial.
The word trilogy was originally applied to a set of three completely separate but interrelated Greek plays. Our best surviving example of these is the Oresteia by Aeschylus about the House of Atreus (the Atreides, in one spelling), which comprised three different plays: Agamemnon, The Libation Bearers, and The Eumenides. The satyr play Proteus, which served as a sort of coda piece (well, or codpiece :) to the trilogy, has not survived.
So that’s three. But what about two?
The word most directly analogous to trilogy is not the neologued duology but rather dilogy. However, that has not historically meant a two-volume set of novels. Per the OED, dilogy is a term of rhetoric meaning:
Etymology: ad. L. dilogia ambiguity, a. Gr. διλογία, f. δίλογος, f. δι‑ twice + ‑λογος speaking. In mod.Fr. dilogie.
The use of an ambiguous or equivocal expression; the word or expression so used.
Repetition of a word or phrase, in the same context. In recent Dicts.
If that ambiguity of sense does not bother you, then go ahead and use dilogy.
Author Dan Simmons often writes what he calls diptychs, stories published in two halves, like two opposing leaves folded into one tale, such as his Ilium–Olympos diptych. Unlike duology but like dilogy and trilogy, diptych is an authentic Greek word.
However, rather than meaning two different words as a putative duology would appear to indicate, diptych means a “two-folded thing”, analogous to a triptych meaning a “three-folded thing”. So a diptych is one piece folded into two halves.
These are not merely two novels occurring in the same world setting; they are two integral halves of the same thing. They might perhaps be called a series in that Ilium is the first half of the singular story concluded in Olympos.
Another example of a diptych by a different author is the The Wizard Knight by Gene Wolfe. These were originally published in two parts, first Knight then Wizard, but the work is now available as one volume.
Another of Wolfe’s works, The Book of the New Sun, has been variously published as four volumes, as two volumes, and as one volume. One might call the two-volume set a diptych were one so inclined, or the four-volume set a tetraptych. But that will probably just get you talked about — and probably more, alas, than the incorrect tetralogy would. Maybe just go for the jocular fourology; at least nobody will ever bother to tell you that that one is wrong. :)
Interestingly, just as Aeschylus had an extra coda to his Oresteia in the lost Proteus, Wolfe relented to his publisher’s pressure by issuing The Urth of the New Sun as a follow-on novel. But Wolfe himself calls it a coda. It is a second novel following the first, and even published as five volumes these do not a pentalogy make.
Perhaps the most notoriously mislabelled example is The Lord of the Rings, a single novel that has seen publication in one single volume, in three volumes, and in seven volumes. No matter how you look at it, The Lord of the Rings is never a trilogy nor heptalogy; it is only a single novel, just like a diptych is. Tolkien himself famously wrote that “of course” The Lord of the Rings was no trilogy, since he actually understood the difference. No matter how many pieces a novel is published in, it is still a novel.
The distinguishing factor is that they make up one contiguous story, and must be read in that order. They might as well have been published as one volume, but for publishing constraints of size and timeliness. On the other hand, the combination of The Hobbit and The Lord of the Rings might be justly called a dilogy, since they are two independent novels that can be read each on their own.
An interesting case containing both example and counterexample is the d’Artagnan Romances, a trilogy by Alexander Dumas. The trilogy’s three novels are The Three Musketeers, Twenty Years After, and The Vicomte of Bragelonne.
That is a legitimate trilogy, as the novels are separate stories. However, the last of those three novels is so large that it has been variously published in sets of three, four, and even five volumes. (You may recognize The Count of Monte Cristo, one of the pieces of the third novel.) Vicomte is all still one novel though, not any sort of whatchamacallogy.
Steven Brust would later mimic Dumas’s structure of a trilogy whose third novel is published in three volumes in Brust’s own Khaavren Romances, an overt homage to the earlier swashbuckling trilogy of Dumas. Brust’s trilogy comprises The Phoenix Guards, Five Hundred Years After, and The Viscount of Adrilankha. However, Viscount was itself published as a triptych of three parts, each under its own title, just as the earlier Viconte had been.
When you have two or three different self-contained stories in the same setting, this is something else. If they are expected to be read in a particular serial order, then they are perhaps a dilogy or trilogy, although it is pointless to the point of obscurantism to keep adding Greek prefixes to ‑logy as further volumes are issued. If you dare call the thirteen Vlad Taltos novels from Jhereg through Tiassa published by Steven Brust through 2011 a triskaidekalogy, you had best be doing so in jest, because no one will take you seriously. They are merely a series, and need be read in no particular order. Another modern series that can be read in any order is the Culture Series by the late Iain Banks.
But when they are just one story that happens to be published separately in a number of volumes, they are not a trilogy nor even a series. They are merely a serial, or a serialized novel. Many novels were originally published this way.
The key point is that a single novel split into three pieces is never a trilogy, nor is a single novel split into two pieces a dilogy or duology.
They are just serialized novels. Words like diptych for a two-parter and triptych for a three-parter are much more accurate than duology or trilogy, since those imply that they are separate novels — separate words, if you will — not merely one thing with two or three folds.
Sometimes you come across larger collections of written works that all occur in the same setting, but may not include the same characters or time periods. Often these contain not just several different series of novels but also shorter works such as novellas, short stories, annals or histories, and sometimes even poetry. Taken as a whole, these are each called a cycle. Taken from a Greek word originally meaning circle, the term was first used in the context of Homeric epics, then of Arthurian ones, and finally found itself applied to later great works of modern writers. The OED defines this sense of cycle as:
A series of poems or prose romances, collected round or relating to a central event or epoch of mythic history and forming a continuous narrative; as the Arthurian cycle. Also transf.
Originally used in the Epic cycle Gr. ὁ(ἑπικὸς) κύκλος, the series of epic poems written by later poets (Cyclic poets) to complete Homer, and presenting (with the Iliad and Odyssey) a continuous history of the Trojan war and of all the heroes engaged in it.
So for example, the Mabinogion is a cycle of eleven stories, the earliest ones we have of the Britons. In a modern context, the tales of Sherlock Holmes by Arthur Conan Doyle are sometimes called the Holmes Cycle, and the P.G. Wodehouse novels and stories about Jeeves and Wooster are occasionally called the Jeeves Cycle.
More recently, the collection of three different series and numerous ancillary shorter works that Gene Wolfe has produced in the setting begun in The Book of the New Sun and followed on in The Book of the Long Sun and The Book of the Short Sun is sometimes referred to as The Solar Cycle, while the two series, one separate novel, and several shorter works that Steven Brust has produced in the Vlad and Khaavren universe is sometimes referred to as The Dragaeran Cycle.
Tolkien preferred to call the aggregate collection of his tales of Middle-earth his Legendarium, a medieval Latin word meaning a collection of legends. To the best of my knowledge, this word is today used exclusively for Tolkien’s world, not for anyone else’s.
Related narratives that are more Nordic in origin or tone are sometimes known as sagas. The Icelandic Sagas collected by Snorri Sturluson are a prototypical example. Modern examples of collections called sagas include Julian May’s Saga of the Pliocene Exiles and Lois McMaster Bujold’s Vorkosigan Saga.