Starrett 81931 Master Square 24 x 24 x 4 5-Face Granite, Grade AA - Laboratory. Five-Face Granite Master Squares are popular 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. By tracing along the top edge of the square while in the vertical position, it will check parallelism of the table in the X and Y axes to the spindle. Five-face master squares may also be used on any work that requires the checking of squareness or parallelism.