A 12-inch grooved iron blade with an attached body forms a fixed 90-degree angle. The body fits into the blade’s groove and is secured by a screw tightened with a nut. When the nut is loosened, the body can slide within the groove, allowing different angles to be set between the blade and the body. A small built-in spirit level in the body helps verify surface alignment. The tool was primarily used by carpenters.