Screw this square fence to any carpenters square and you'll no longer need to tip the fence on your square down to reference against the project piece edge. It sits perfectly in line with the edge of your carpenters square. The key is that if your carpenter's square isn't perfectly square, you now have a way to correct that. Just loosen the screws, adjust for perfect alignment and tighten the screws again. the fence is 8" long and made of anodized aluminum.