This material is based upon work supported by the NSF National Center for Atmospheric Research, a major facility sponsored by the U.S. National Science Foundation and managed by the University Corporation for Atmospheric Research. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of the U.S. National Science Foundation.

Regardless of the job being undertaken, the first step when using a combination square is to check the square to ensure its accuracy. This task can be easily accomplished by first taping a white piece of paper to a board with a perfectly straight edge. Place the anvil of the square against this edge and draw a line on the outside edge of the blade. Once this is complete, turn over the square and draw a second line at least 1/32 in. away from the first line drawn. If the two lines drawn are parallel with each other, then it can be said that the combination square is accurate.

A combination square is a multi-use measuring instrument which is primarily used for ensuring the integrity of a 90° angle, measuring a 45° angle, measuring the center of a circular object, find depth, and simple distance measurements. It can also be used to determine level and plumb using its spirit level vial. If you need a guide on how to use a spirit level, take a look at our information page. The versatility of the combination square insures its dominance over any other square in its class.

To test the accuracy of the shoulder, one may simply gauge the angle it makes with the blade against a 45° drafting triangle. If the angle between the shoulder and the blade is the same as that of the triangle, then the square is accurate.

Before using a combination square, one must first be familiar with the structure and terminology of the square itself. A combination square consists of a rue-type blade attached to a handle. The handle is composed of two parts, a shoulder and an anvil. The shoulder is placed at an angle of 45° between itself and the blade. It is therefore used for the measurement and layout of miters. The anvil is placed at a 90° angle between itself and the blade. The handle contains an adjustable knob which allows it to move freely horizontally along the edge of the ruler so that it may be tailored towards any size job. Additionally, contained within the head of the handle is a scriber used for marking measurements and a vial which may be used for measuring plumb and level.