Now, when it's drawn out like this, it's pretty clear that our left Riemann sum is going to be an overestimation. Why do we know that? Because these rectangles, the area that they're trying to approximate are always contained in the rectangles, and these rectangles have this surplus area, so they're always going to be larger than the areas that they're trying to approximate. And in general, if you have a function that's decreasing over the interval that we care about right over here, and strictly decreasing the entire time, if you use the left edge of each subdivision to approximate, you're going to have an overestimate, because the left edge, the value of the function there, is going to be higher than the value of the function at any other point in the subdivision. And so that's why, for a decreasing function, the left Riemann sum is going to be an overestimation. Now, let's think about the right Riemann sum, and you might already guess it's going to be the opposite, but let's visualize that. So let's just go with the same three subdivisions, but now let's use the right side of each of these subdivisions to define the height.