A 12" long grooved blade made of iron to which a body is attached, the combination of which creates a 90-degree angle. The additional body is attached to the ruler inside the groove and is fixed with a screw that is tightened with a nut. Loosening the nut frees the body to move within the groove and allows corners at different angles to be created between the ruler and the body. Inside the body there is a tiny spirit level that allows you to verify the alignment of the surface you are working on. It was mainly used by carpenters.