The fifth-wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. The coupling consists of a kingpin, a 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. 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. The advantage of this type of coupling is towing stability. Some camper trailers also use a fifth-wheel configuration, with the coupling installed in the bed of a pickup truck as a towing vehicle; "fifth wheel" is therefore sometimes used as a synonym for such campers. Origin 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 facilitate turning. A wheel would be placed on the rear frame section of the truck, which at the time had only four wheels, making the additional wheel the "fifth wheel". The trailer needed to be raised so that the trailer's pin would be able to drop into the central hole of the fifth wheel. Fifth wheels were originally not a complete circle and were hand forged. When mass production of buggy parts began in the early 19th century, fifth wheels were among the first products to be made. There were a number of patents awarded for fifth-wheel design. Edward and Charles Everett, Quincy, Illinois patented a type of fifth wheel in 1850, followed by Gutches' metallic head block and fifth wheel in 1870 and Wilcox fifth wheel in 1905. Steam Tractors with Fifth-wheel A Thornycroft steam tractor with articulated trailer competed in the 1898 heavy vehicle trials in Liverpool which took place from May 24 to May 28. Built by the Steam Carriage and Wagon Company of Chiswick, the trailer and tractor unit were connected by a turntable though the details of the coupling are not revealed apart from a schematic. Another early example of the fifth wheel in a motorised vehicle was the Messrs