Armstrong 39-020D 1/2-Inch Drive 6 Point Standard Socket, 20mm Made in the USA - quality and availability Radius corner design engages the flats of the fastener, not the corners, providing up to 15-Percent -20-Percent more torque while reducing fastener rounding, enhancing the turning of worn fasteners EYE-D markings optimize size visibility, also providing SAE / metric differentiation Unique socket retention method, cold form ball recess, provides consistent secure retention of the socket to the drive tool eliminating the possibility of either a loose fit or tight fit Socket wall dimensions are optimized to provide strength while retaining access to restricted fasteners