9-12-3 AA-Grade Granite Tri-Square 3-FaceAA Grade Granite Tri Square 3 Face 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 90 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