3 1.5 inch plywood doesn't actually measure 3 1.4 inch. Take a look at that. Okay, so here's the simple solution whenever you're building anything out of plywood. Take two of the parts and hold them together. It's usually a good idea to clamp them just so they don't move around too much. There you go, just like that. And now again, let's say this box is going to be 13 inches wide. You hold the 13 inch line, the graduation line right on the outer edge, and you read the inner edge. There you go. 11 and 9 16th, not 11 1.5. So you cut these pieces, the interior pieces, to 11 and 9 16th. In this box we'll measure exactly 13 inches.