Used mainly in machining and woodworking settings whenever a measured pattern must be maintained, a double square is a tool of measurement used to measure square angles in multiples of two. This is made possible by the part of the tool that allows for the movement of one of the two main parts of the double square: the sliding blade. These tools appeal to machinists who specialize in precision parts, as well as to woodworkers for use in small-measurement projects. Any person who specializes in patternmaking, without a doubt, uses a double square in doing so.
The two main components of the tool are referred to as the rule and the blade. The rule is generally a tempered ruler, made out of a durable metal and most commonly 12 inches (30.48 cm) long with a groove or a rail running the length of it. The ridge, rail, or groove depends on the preference of the manufacturer, but in every case must be perfectly parallel with the long edges of the rule. It also is responsible for acting as a guide whenever the blade needs to be adjusted or moved along the length of the ruler for proper measurement.
The blade is the moving part in a double square and is most commonly manufactured with a mechanism to secure the blade against the rule. This securing device is typically a hook or notch that usually coincides with the placement of the groove or rail on the rule. The rule guides the blade in a straight manner against itself, and when the proper measurement is reached on the rule, the blade is secured in place, usually by a screw or clamping device. The blade is generally made out of a tempered steel, which while heavy also offers stability and durability — necessary aspects of a precision tool.
Double squares can be made out of different materials: aluminum, steel, titanium or different alloys that can be used to produce a square that will last and endure the abuse that may occur in a shop environment. Some are quite simple mechanisms, while others boast a couple of accessories, depending on the application the tool will generally be used in. Bubble levels are popular add-ons to the blade of the tool, as are magnetic strips that help secure the double square to certain materials that are being worked with.