Say you have an awesome book—but it’s long. Too long. So you cut it in half: now you have two books. Easy enough, right? For fixing page count, sure! But when it comes to the enjoyment factor of the book itself, it can have a remarkably harsh effect. See, in a traditional story arc, the climax—or peak of the arc—is pretty close to the end of the book. In between the beginning and that climax, there’s a whole bunch of stuff, like the introduction to the characters and setting, the inciting incident, and a ton of obstacles the characters have to overcome—including usually one spectacular failure--in order to grow and achieve their goals.
So, if you cut the book off half-way that means your stunted plot gets through the introduction, the inciting incident, and maybe a problem. But the tension is only starting to ramp up at the very end of the book, there isn’t a climax, and you’ve not generally hit that critical failure point yet. Meaning, unless you’re really careful, your book is going to feel slug-slow and crazy unbalanced—even if it’s perfect as one huge book.
This isn’t to say that you can’t start at different points in the traditional arc, that you can’t have several arcs going at the same time, or that you can’t have a completely nontraditional arc. You totally can! In fact, those things often make for some of the most popular books and movies. But, that being said, it’s important to know why each of those pieces of the arc pie are there—what purpose they serve, and how they work with the other pieces—before deconstructing the whole pie concept and making crazy substitutions.
So, to those ends, here are some of the basics on the average story structure—what each piece does, and what you need to provide if you’re planning on cutting it out or changing it. You can also use this list as a basis for an outline, making sure you have each piece of this basic puzzle in place before filling out all the juicy details.