In my sample here I need a 3 1/2 inch diagonal half square. Instead of cutting 3 7/8 inch squares, I started with 4 inch squares. I used my quick quarter to mark it, sewed it, trimmed the dog ears and then pressed. However, I do not have the perfect 3 1/2 inch diagonal half square I need, but one which must be trimmed back. I will carefully trim this larger square back to the size I need. I will not trim just one side, but trim carefully on all four sides. I'm going to use the 45 degree line of my ruler on the diagonal sewn line to keep the block a true diagonal half square. Half of the 3 1/2 inches I need is 1 3/4, so I will use that mark on my ruler as a guide to keep the square centered. (If you have trouble finding the true center, just fold it and finger press it to mark the center.)

