5th Wheel Kingpins
The coupling consists of a kingpin, a 2-or-3 1⁄2-inch-diameter (50.8 or 88.9 mm) steel pin on the front of the semi-trailer, and a horseshoe-shaped coupling device called a 5th wheel on the rear of the towing vehicle.
The term 5th 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. Basically a wheel was placed on the rear frame section of the truck, which back then only had four wheels; this wheel that was placed on the frame was the "5th wheel", hence the name. The trailer needed to be raised so that the trailer's pin would be able to drop into the central hole of the 5th wheel.
Today's 5th wheels allow the trailers to slide into the 5th wheel and lock into it, and are a very reliable unit when maintained and serviced properly. The engagement of the kingpin into the 5th wheel locking mechanism is the only means of connection between tractor and trailer; no other device or safety mechanism is used. Couplers and pintle hooks use safety chains in the event of a trailer separation while going down the road. Trailer-to-trailer connection can also be made by using 5th wheels.