Abstract: A geometric instrument useful as a carpentry tool. The invention is a square of thick aluminum plate in the shape of a right isosceles triangle with a handle fitted and perpendicular to the hypotenuse. A rabbet cut is made in the edges of equal sides of the triangular carpenter's square. A second embodiment employs adjustable feeler bars in place of the rabbet cut for checking the square of window and door jambs. Also, the miters or feeler bars facilitate the laying out of short points on the jams for miter cutting.