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The Breaks: Why Big Rigs Are Called "Semis"

Have you ever wondered why there are so many names for the 18-wheeled tractor-trailers traversing America’s highways? Popular nicknames like big rig, semi, and  18-wheeler are all accurate descriptions of these highway monsters.  The origin of “semi,” one of the most commonly heard slang terms, stems from the actual hitch that holds the tractor and trailer together while on the road.
There are two types of hitches, drawbar and semi. First off, the abbreviation “semi” comes from “semi-oscillating turntable hitch.” A turntable hitch is commonly known as a “5th wheel” or “5th wheel coupling.” A semi hitch connects to and sits on a greased turntable-like structure on top of the truck’s rear axle and adds more than one axle to the truck.  The drawbar trailer is commonly seen when used by a four-wheel motorist, usually when pulling a personal boat, or small trailer.  A drawbar trailer hitch attaches to a four-wheel vehicle via a ball hitch and provides one more axle to the vehicle.
A semi-oscillating turntable hitch allows oscillation on two planes, horizontal rotation (yaw) and  up or down rotation (pitch).   The semi hitch attaches to the truck both as a stationary hitch as well as an adjustable hitch. The adjustable hitch known as a rack system, gives the driver freedom to determine the distribution of weight on his truck. The hitch moves vertically on a rack and is then locked into place to ensure the perfect proportion of the load’s weight.
A fully-oscillating turntable hitch is less common, but allows for every  possible rotation–roll, yaw, and pitch. The fully-oscillating hitch works best with tankers. During travel, twisting can damage the tank if a fully-oscillating turntable isn’t used. This hitch is also used for off-road traveling vehicles such as logging trucks, where the road service is very uneven. The fully-oscillating hitch is more complex (causing it to be more expensive) and is not important most of the time (a little twisting along a trailer’s length doesn’t hurt most trailers).

All in all, the semi-oscillating hitch is the most common hitch on the road, which gives the slang term “semi” some credibility.