Ever wonder why every contractor you see has a triangular square stuck in his tool belt? This cheap, handy little tool is called either a rafter square or a speed square, and it can offer you the biggest bang for the buck in your toolbox. This tool has dozens of uses; here are three of our favorites.

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Speed squares aren't just for marking 90- and 45-degree angles when you're cutting 2x4s. You can use a speed square to find roof pitches, guide your circular saw and more—including marking any angle from 0 to 90 degrees.