When it comes to teaching students how to make sure your foundation has a true 90 degree corner to work from, I like to use the simple "3, 4, 5 Rule." This is also known as the Pythagorean Theorem (A^2 +B^2 = C^2). This is truly a great and easy way to assure a square foundation. This is also a good way to reintroduce the basic geometry that they learned in math class. Now they will have a chance to use it in a practial setting or real life.