So the left boundary of the nth rectangle is going to be x sub n minus 1. Whatever the rectangle number is, the left boundary is x sub that number minus 1. And this is just based on the convention that we've defined. Now the next thing that we need to do in order to actually calculate this area is think about what is the width. So let's call the width of any of these rectangles. And for these purposes, or the purpose of this example, I'm going to assume that it's constant. Although you can do these sums where you actually vary the width of the rectangle, but then it gets a little bit fancier.