Wikipedia
Combination square
A combination square is a tool used for multiple purposes in woodworking, stonemasonry and metalworking. It is composed of a ruled blade and one or more interchangeable heads that may be affixed to it. The most common head is the standard or square head which is used to lay out or check right and 45° angles. Invented in 1883 by Laroy S. Starrett, the combination square continues to be a commonplace tool in home workshops, construction jobsites and metalworking.