steel square
A steel square, also commonly referred to as a framing square or carpenter's square, is a tool used in carpentry. It is a long-armed square (a tool used for marking and referencing a 90° angle) consisting of a wider arm and shorter, narrower arm, that meet at a right angle. Traditionally made of steel, they're also commonly now made of other materials such as aluminum.
The longer wider arm is two inches (51 mm) wide, and is called the blade; the shorter narrower arm, is one and a half inches (38 mm) wide, and is called the tongue. The square has many uses, including laying out common rafters, hip rafters and stairs. It has a diagonal scale, board foot scale and an octagonal scale. On the newer squares there are degree conversions for different pitches and fractional equivalents.