A square is a tool used to mark and reference 90° angles, although miter squares are used to measure 45° angles. They are commonly used in carpentry, metalworking, construction and technical drawing.[1] Some squares incorporate a scale to measure distances (a graduated ruler) or to calculate angles.

Many squares are made of two parts, a "stock" and a straight "blade" or "tongue".[1]​ The stock is usually thicker than the blade, which is fixed in or on the stock. The blade usually has parallel edges. The stock is usually held against the edge of a workpiece or drafting table and then the tang is used as a ruler to make a mark or as a reference to check the accuracy of an angle.

Wooden checking squares have been preserved from ancient Egypt and ancient Rome, and are represented in works of art from both periods. Starting in the century, they began to be produced in factories, although previously it was the same workers who built their own squares,[1]​ and some carpenters continue to do so.[2]​.

There are several different types of squares that are used for a variety of different industries and purposes:.

Different industries and applications have different precision requirements for squares. Some are manufactured to meet regulated precision standards, such as British Standards BS 939:2007 for engineering squares and BS 3322 for test squares.[23]​[24]​.

A square can become inaccurate over time due to common use or deterioration, such as when the edges become worn, the square is dropped, or abused. Materials, especially wood, can also vary with changes in temperature and humidity. The accuracy of squares can be checked with a known reference, such as a square or piece of machinery that is known to be accurate, or with a cylindrical square. A bright light or feeler gauges can also be used to check for gaps between the square and the referenced object.