1. a series or group of three plays, novels, operas, etc., that, although individually complete, are closely related in theme, sequence, or the like.
2. (in ancient Greek drama) a series of three complete and usually related tragedies performed at the festival of Dionysus and forming a tetralogy with the satyr play.
That means that each book can stand alone. The reader isn't left with that dangling feeling of "Wow...there's only five more pages and a whole lot to wrap up...." Or worse, the reader turns the final page and gapes at the last sentence. Say what?
Authors who want to write trilogies need to understand that readers want a satisfying conclusion to Book One, not a cliff hanger to Book Two. (Click to Tweet!) And if there isn't going to be a satisfying conclusion, then you need to let them know up front that Book One is not the first of a trilogy, but the first of a Three-Decker, or three-volume novel.
You can read up on why and how trilogies started here. In short, Tolkien is given a lot of credit for starting the trend (though it existed long before), and poor Tolkien is the worst example, because he never intended his book to be split up. It was only published in three parts because of economic reasons ($ of paper at that time, more precisely). The Lord of the Rings is actually a perfect example of a Three-Decker.
For you visual learners out there, here's the breakdown of the relationships between the three books in a trilogy and in a three-decker:
Each book stands alone, but is related to the other two, whether this be through similar characters or themes. A common method is that secondary characters in Book One become primary characters in Book Two. Each book is not contingent on the others to get a full story line or character arc. They each have satisfying conclusions. There are NO cliffhangers.
Each book moves toward a final conclusion. Books Two and Three aren't readable as stand-alone novels. They build off of each other, with Book Two picking up right where Book One left off, and Book Three picking up where Two left off. They are major cliffhangers which send you to Amazon immediately to check out if the next book is available. If it is, you're in frenzied delight. If you have to wait another 9-12 months, you end up frustrated and unfulfilled.
There's nothing wrong with either literary means of delivering your book to the world. Publishers often use the wrong terminology to tell readers both of the above scenarios are trilogies. You can hardly expect a publisher to slap a sticker or graphic on the cover saying, "Installment One" or something similar. If it tanks, then they aren't on the hook for more.
Perhaps this is just one of my pet peeves? Maybe no one else out there feels the same way. Many dear friends have written Three-Deckers, calling them trilogies, and frustrating me with having to wait for the rest of the installments...sometimes up to a year! Had I known up front, I would wait until the whole thing is completed, so that I get a satisfying conclusion.