Your geometry teacher would be proud
Long before the invention of modern clamps and jigs, people used their intellect to create square corners. They did this by using the Pythagorean theorem, a geometric proof in which the square of the longest side of a right triangle equals the sum of the squares of the remaining sides. Woodworkers, carpenters, and engineers use this method to create square corners when dimensions exceed the realm of a standard framing square. By using a triangle with sides in exact multiples of 3, 4, and 5 (for example: 6, 8, and 10), you'll always get a square corner opposite the longest side.