A combination square is a type of square commonly used for layout work. The combination square has a 12-inch blade that moves through a head. The head is marked with 45-degree and 90-degree angle measures. Some squares also contain a small spirit level and a carbide scriber, which is a sharp, pointed tool for marking metal. Among the tasks for which a combination square is well suited are: testing work for squareness, marking 90-degree and 45-degree angles, checking level and plumb surfaces, and measuring lengths and widths. It can also be used as a straightedge and marking tool.
Good combination squares have all-metal parts, a blade that slides freely but can be clamped securely in position, and a glass tube spirit level that is truly level and tightly fastened.