I wrote a trilogy once. It says so on the Internet. It's called The Truth Sayer.
I did it by accident. The Truth Sayer was meant to be a series, but a sad lack of action on the behalf of the book-buying public meant it only got as far as three books before the publishers pulled the plug.
Does that make The Truth Sayer a trilogy?
Well, let's put it this way: if I'd known to start with that there were going to be only three books then they'd have been different.
The same sort of thing happened with Mervyn Peake's Ghormenghast trilogy, though the reason there are only three Ghormenghasts is that the poor man got too ill to continue.
And there are even less accurately-named trilogies. The sublime Hitchhiker's Guide To The Galaxy trilogy contains five books. The also-sublime Lord of the Rings trilogy isn't a trilogy at all, but one long story originally published in three volumes.
Wagner's Ring Cycle trilogy (which is a trilogy of operas) has four works in it, but people say the first opera doesn't count because it only lasts for slightly over three hours, unlike the others, which each lasts pretty much forever.
So, look, do you know something? I wish that people would make their flipping minds up about what a trilogy is, because it's dead annoying to read the first book in a trilogy and then after three hundred pages find the hero is left in a rat-infested cellar with poison gas hissing in through the air vent and the sound of maniacal laughter ringing in his ears.
Trilogy: that's three self-contained but connected works of art which form a complete whole.
If it's not that, call it something else.
Word To Use Today: trilogy. This word comes from the Greek trilogia, which meant three plays performed together at a festival. The logy bit means word.