Abstract: A king pin for a tractor trailer coupling is disclosed. The king pin is substantially conventional except that the lower flange thereof, which is presently standardized to have less diameter than the upper part of the king pin, is made predeterminedly larger than the upper part so that it cannot be received in the usual recess at the upper end of the coupler jaw when closed, or when the jaw is opened the flange positively prevents the jaw from closing. Thus a trailer can never be towed at all by partial seating of the flange of a misaligned king pin in the upper recess of a closed jaw as now can occur with a standardized king pins resulting in unexpected and dangerous separation of the tractor and trailer on the open highway.