A combination square is a multi-purpose measuring and marking tool used in metalworking, woodworking, and stonemasonry. It is composed of a rule and one or more interchangeable heads that can be attached to the rule. Other names for the tool include adjustable square, combo square, and sliding square. The most common head is the standard head, which is used as a square for marking and testing 90° and 45° angles. The other common types of head are the protractor head, and the centre finder head.