This twist socket set is specially designed for removing rounded nuts bolts and studs. As you turn the socket anti clock wise the spiralled cutting blades grip and remove the nut or stud. Sizes: 17, 19, 21 and 22mm Drive: 1/2" Square With 12mm straight bar Made from CR-MO Protective glasses must be worn when using this product Keep sockets free from dirt and solvents to prolong life.