In the U.S., during the transition from U.S.S. standard to S.A.E. standard, having the S.A.E. mark (Society of Automotive Engineers) was a clue what type of bolt / nut the wrench was designed to fit. U.S.S. size designations were based on shank diameter of the bolt; S.A.E. size designation on the width of the bolt head / nut.