The fifth wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. Some recreational vehicles RVs are in a fifth wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle. The coupling consists of a coupling pin (or king pin) on the front of the semi-trailer and a horseshoe-shaped coupling device called a fifth wheel on the rear of the towing vehicle.
The term fifth wheel comes from a similar coupling used on four-wheel horse-drawn carriages and wagons. The device allowed the front axle assembly to pivot in the horizontal plane, to improve turning.
It was invented in the UK by Clive Henderson