A method, based on the Pythagorean Theorem, of squaring a foundation layout. Points are marked 3 feet from the corner on one side and 4 feet on the other. If the angle is truly 90 degrees, a line connecting the two marked points will measure precisely 5 feet.

A lender uses this rule to determine a borrower's housing affordability. It says that a borrower can afford no more than 35% of monthly take-home pay.