SEMI TRUCK AND TRAILER DIMENSIONSA tractor trailer is the combination of a tractor unit and one or more semi-trailers designed to carry bulk freight. A semi-trailer attaches to the truck with a type of hitch system called a fifth-wheel. The fifth wheel is permanently attached to the semi truck. The king pin is a part of the semi trailer. The truck backs the fifth-wheel under the trailer, lining it up with the king pin until it locks into place.
There are many names for tractor trailers: a transport truck, semi-truck, trailer truck, tractor truck, truck tractor, transfer truck, articulated truck, semi-trailer, tractor-trailer, semi-tractor, tractor, sleeper cabs, big rig, and of course eighteen-wheeler.