Stanley Proto JSBV1618 Double Box Racheting Spline Wrench 1/2 X 9/16 Spline fasteners help tighten high strength bolts which are used in high torque load applications. Spline helps reduce foreign object debris (FOD) by having a precise fit between the fastening tool and its fastener. Black chrome finish is corrosion resistant and visually distinctive. Ratcheting spline box end is countersunk for easy application to the fastener. The fractional spline wrenches can be used on fractional 4, 6, 12 point and spline fasteners.