The circular forms of wire gauge measurement devices are the most popular, and are generally in diameter, with thirty-six notches; many have the decimal equivalents of the sizes stamped on the back. Oblong plates are similarly notched. Rolling mill gauges are also oblong in form. Many gauges are made with a wedge-like slot into which the wire is thrust; one edge being graduated, the point at which the movement of the wire is arrested gives its size. The graduations are those of standard wire, or in thousandths of an inch. In some cases both edges are graduated differently in order to allow comparison between two systems of measurement. A few gauges are made with holes into which the wire has to be thrust. All gauges are hardened and ground to dimensions.