Starrett 81967 Tri-Square 18 x 24 x 4 3-Face Granite, Grade AA - Laboratory. Three-Face Granite Tri-Squares provide an excellent, economical way for accurately checking the X-Y-Z axes on CNC machine tools and coordinate measuring machines. Lying in the horizontal position, the X and Y axes can be checked for 90º squareness. With the square in the vertical position, tracing along the vertical edge of the square can check the perpendicularity of the Z axis. Granite tri-squares may also be used in the same manner that steel squares would be used for the direct checking of squareness and straightness.