framing square.
A framing square is used to cut stairs, rafters, or anything else requiring repeated cuts that are square or of a given angle. How do you use it? Depends on what you are doing....
One-third is the same as five-fifteenths. Two-fifths is the same as six-fifteenths. Five-fifteenths plus six-fifteenths is eleven-fifteenths. One, or fifteen-fifteenths, minus eleven-fifteenths is...