In order to answer this question, it is necessary to answer to other questions:
- What does "trilogy" mean?
- Does Tolkien's The Lord of the Rings fit that definition?
Chris Baldick's Concise Oxford Dictionary of Literary Terms (second edition, 2001) defines "trilogy" as
a group of three connected plays or novels. (…) There are several examples in modern prose fiction, including Samuel Beckett's trilogy of novels, Molloy (1950), Malone Meurt (Malone Dies, 1951), and L'Innombrable (The Unnamable, 1952).
"Connected" suggests a looser connection that a continuous narrative. An older example given by Baldick is Shakespeare's Henry VI plays, which were probably not written in the order suggested by their modern titles (Henry VI, Part 1 was probably written after Henry VI, Part 2). Baldick's examples include both plays and novels.
A Handbook to Literature by William Flit Thrall and Addison Hibbard (1936) defines "trilogy" as follows:
A literary composition, more usually a novel or a play, written in three parts, each of which is in itself a complete unit. Shakespeare's King Henry VI is an example. The trilogy is written against a large background which may be historical, philosophical, or social in its interests.
"Written in three parts" suggests more coherence than Baldick's definition. However, Shakespeare's Henry VI, was probably not planned in the order we read those plays today and there is no evidence that the original audience first saw these plays in the order suggested by their modern titles. It is definitely not a play in three parts.
J. A. Cuddon's Penguin Dictionary of Literary Terms and Literary Theory (third edition, 1991) first gives a "definition" that does not suggest any connection based on context:
A group of three tragedies presented by individual authors at the drama festival in Athens in the 5th C. BC.
Aeschylus's Oresteia is the only example from that time that has survived (as Baldick also acknowledges). The three plays in this trilogy are connected because they all concern the house of Atreus and through common themes, especially revenge. Since no other examples have come down to us, it is not clear whether this type of continuity was a requirement for Athenian theatre trilogies.
Cuddon gives more recent examples: Shakespeare's Henry VI, Friedrich Schiller's Wallenstein (about the decline of Albrecht von Wallenstein), Eugene O'Neill's Mourning Becomes Electra (three plays that are normally not produced individually) and Arnold Wesker's Chicken Soup with Barley, Roots and I'm Talking About Jerusalem (three plays which can stand on their own).
Cuddon then adds,
The term may also be applied to a group of three novels linked by a common theme and characters. A good modern example is Joyce Carey's Herself Surprised, To Be a Pilgrim and The Horse's Mouth (1941–4).
The Horse's Mouth was adapted into a film in 1958, independently of the two other novels.
The entry trilogie in the Dutch Algemeen letterkundig lexicon points out that the trilogy proper can be distinguished from the novel in three parts.
The above definitions indicate ambiguity in the term, since it can refer both to
- a group of three works that can each stand on their own (Becket's "trilogy", Arnold Wesker, Joyce Carey), and
- (perhaps less commonly) a longer work that presents a continuous story (Wallenstein, Mourning Becomes Electra).
However, none of the examples given above is actually a novel in three parts or volumes.
How does this apply to J. R. R. Tolkien's Lord of the Rings? In "Note on the Text" to the 2004 edition, Douglas A. Anderson wrote,
J. R. R. Tolkien's The Lord of the Rings is often erroneously called a trilogy, when it is in fact a single novel, consisting of six books plus appendices, sometimes published in three volumes.
It makes no sense to read The Two Towers without first reading The Fellowship of the Rings, or to read The Return of the King without reading the first two parts. This distinguishes The Lord of the Rings from the novel trilogies by Becket and Carey listed above. The Lord of the Rings is not a trilogy in the sense that it is not a trilogy of three novels that can be read on their own.