For accurate setting of let off by section. Includes two polyethylene end blocks and one aluminum starter rail. The gauge is assembled by inserting the rail into the end block slots. The gauge is then placed on the hammer rail behind the shanks and positioned to shim the hammers to the point of let off. Full instructions describe a tapping procedure to achieve accurate results quickly, without eye fatigue. See video demonstrations parts 1, 2, and 3 here.