Abstract: An anti-jackknifing coupling device, for an articulated veicle such as a tractor-trailer combination, comprising a base member for mounting upon the tractor. A turntable member is rotatably mounted upon the base member, and a trailer support member is supported upon, and rotatable with, the turntable. The trailer support member, usually called a "fifth wheel", serves to releasably couple the trailer. A piston-and-cylinder device is connected at its one end to the turntable and at its other end to the base member, so taht it extends substantially diametrically across the turntable member when the vehicle is straight. As the tractor and trailer pivot, the trailer support member and turntable rotate together relative to the base member, causin contraction of the piston-and-cylinder device. The trailer support member will usually be able to tilt relative to the turntable about a lateral pivot axis.