A try- square is a woodworking tool used for marking and measuring a square piece of wood. The square refers to the tool's primary use of measuring the accuracy of a right angle (90 degree angle); to try a surface is to check its straightness or correspondence to an adjoining surface. "Try square" is so called because it is used to "try" the squareness.
A piece of wood that is rectangular, flat, and has all edges (faces, sides, and ends) 90 degrees is called four square. A board is often milled four square in preparation for using it in building furniture. A traditional try square has a broad blade made of steel that is riveted to a wooden handle or "stock". The inside of the wooden stock usually has a brass strip fixed to it to reduce wear. Some blades also have graduations for measurement. Modern try squares may be all-metal, with stocks that are either die-cast or extruded.