If you don't have a square, here is a neat trick I learnt today. measure 6 inches on the inside of the bottom rail from where the rails join, and mark a line. Then measure 8 inches on the inside of the "perpendicular" rail from the base rail. Put your tape or ruler on the six inch mark, and measure up in a direct line to the 8 inch mark. It should measure 10 inches if you are square.