A combination square is a versatile tool used by carpenters, metalworkers, and other craftspeople to measure and mark straight lines, check the accuracy of angles, and check the flatness of surfaces. It consists of a ruler, a level, and a 45-degree angle.
From Wikipedia
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. The tool is also known by the names adjustable square, combo square, and sliding square.