miter square

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An instrument with straight edges that are set at a 45° angle or that are adjustable, used for marking the angles of a miter joint.

GNU Webster's 1913

1. n. (Carp.) a bevel with an immovable arm at an angle of 45°, for striking lines on stuff to be mitered; also, a square with an arm adjustable to any angle.