<P> The fifth - wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly . Some camper trailers use a fifth - wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle, and "fifth wheel" is therefore sometimes used as a synonym for such campers in North America . The coupling consists of a kingpin, a 2 - or - 3 ⁄ - inch - diameter (50.8 or 88.9 mm) vertical steel pin protruding from the bottom of the front of the semi-trailer, and a horseshoe - shaped coupling device called a fifth wheel on the rear of the towing vehicle . </P> <P> As the connected truck turns, the downward - facing surface of the semi-trailer (with the kingpin at the center) rotates against the upward - facing surface of the fixed fifth wheel, which does not rotate . To reduce friction, grease is applied to the surface of the fifth wheel . The configuration is sometimes called a turn - table in Australia and New Zealand, especially if it is a rotating ball - race - bearing type . </P>

Where did the term fifth wheel come from