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.