They don't have to be, but let's say we do. So the first one would go from two to four, the next one would go from four to six, and the next one would go from six to eight. If we do a left Riemann sum, you use the left side of each of these subdivisions in order to find the height. You evaluate the function at the left end of each of those subdivisions for the height of our approximating rectangles. So we would use g of two to approximate for, or to set the height of our first approximating rectangle, just like that, and then we would use g of four for the next rectangle. So we would be right over there, and then you'd use g of six to represent the height of our third and our final rectangle right over there. Now, when it's drawn out like this, it's pretty clear that our left Riemann sum is going to be an overestimation.