A speed square has many obvious uses for marking up your building materials. Here's a small hidden feature you may not know about: it can also draw perfect circles.
Photo by HomeHacks
Most speed squares have a notch at the 90 degree corner which is used to determine angles. To draw a circle using this notch you just need to place a nail at the centre of your desired circle and then put your notch on the nail. Using a pencil, walk the speed square around the nail to draw a circle. Neat, hey?
Draw Circles Without a Protractor [HomeHacks]