Abstract: A tractor-trailer is provided with a jam pin carried by the fifth-wheel coupler which is adapted to be projected up into a jam plate carried by or comprised by the wear plate or weld plate of the semi-trailer which has a slot therein of such length as to limit the angle through which the rig can be articulated to prevent or minimize jackknifing. The jam pin is controlled by control means in the cab so that, when jackknifing is an imminent possibility, the driver can throw the jam pin into operative position and thus eliminate or minimize the possibility of jackknifing.