A combination square is a tool used for multiple purposes. The Square refers to the primary use of the tool: measuring the accuracy of a right angle (90°).

At its minimum, the tool is composed of primarily two parts: a rule and a head.

Uses

• Measuring angles — A combination square can be used to measure a 90° and a 45° angle. The 45° angle is used most commonly in miter cuts.
• Determining flatness — When working wood, a first step is to develop a reference surface on a board. From this reference surface, all else in the workpiece is measured. The rule of the combination square is assumed to be flat.
• Measuring the centre of a circular bar or dowel. The rule is assembled through the center of the center square, the two cast iron legs of the center square are then placed against the outside of the bar (dowel) allowing a center line to be scribed alongside the ruler. Perform this action at two locations and the intersecting lines will approximate the center of the bar (dowel).
• Protractor head allows angles to be set and measured between the base and ruler
• A rudimentary level for approximating level surfaces is incorporated in the protractor and also the 45° holder.

In woodworking, the starting raw material is neither flat nor square, however, the end product (e.g. a table) must be flat and have corners and legs that are square. Woodworkers utilize a reference surface to create and measure flatness. Their workbench is usually considered a reference surface (thus flat) and can be used to initially create a flat surface. However, the combination square is a more precise measurement and is used in these initial steps of woodworking to create and mark a reference surface and reference edge from which all other forms of the work will be measured from and must rely upon.

The protactor is ideal in metal work for setting large items at the required angle in machine vices. The long reach of the ruler and firm, heavy base aid the setting up process.