Well, I’ve seen a few driven by half-wits.

Dear Word Detective: The simple wooden box mounted on two wheels and pulled behind a car to transport bulky and/or heavy objects from the hardware store is known as a trailer. The huge and frightening metal box mounted on 400 wheels and pulled behind an equally huge diesel truck is known as a semi-trailer. Something doesn’t add up. Why is the larger one considered only half of the smaller one? — Jim Brown.

That’s a good question, but before we begin I should note that “big rigs” of the sort you’re talking about usually have eighteen “wheels” (actually tires), not 400. I actually tried to learn to drive one of those things once, and I did OK going forward, but I was absolutely flummoxed by trying to back up to a loading dock. By the way, there is nothing quite like unloading 30-pound bundles of newsprint from a 40-foot steel trailer — by hand — on a lovely August afternoon. It’s like doing push-ups in a microwave oven. Yes, I’ve had some really awful jobs.

“Semi-trailer trucks” (aka “semis”), also known as “tractor-trailer trucks” and, apparently, “articulated lorries” in Britain, have been in use since the early 20th century, but only became really common in the US with the development of the interstate highway system after World War II. The usual configuration of these trucks is a long trailer with eight tires on two axles at the rear of the trailer, pulled by a “tractor” truck unit with ten tires (two axles of four at the rear, one with two in front). Incidentally, a tractor driven with no trailer attached is called “bobtailed,” and is usually a very bumpy ride.

The root sense of “trailer” is “someone who follows a trail” or, more relevantly, “something that is dragged along behind.” The verb “to trail” comes ultimately from the Latin “trahere,” meaning “to drag, draw, pull or haul.” (That “trahere,” incidentally, also gave us the English word “tractor.”) The use of “trailer” to mean an unpowered vehicle towed by a car or truck dates back to around 1890. Interestingly, “trailer” in the sense of “short promotional excerpt from a movie” dates back to 1928. They were originally shown after, not before, the main feature, thus the name.

There are all sorts of trailers in use, from the small cargo trailers you rent and hitch to your car to the sort of “Long, Long Trailer” that gave Lucy and Desi such grief in the 1953 movie of that name. Strictly speaking, a “full trailer” has both front and rear axles and is simply pulled behind the towing vehicle. But the trailer of a semi-trailer truck has no front wheels. The front of the trailer (and about half the trailer’s weight) rests on the rear of the “tractor” (locked in a peg-and-collar gizmo called a “fifth wheel”). The trailer swings separately from the tractor in turns, but in most respects you’re dealing with a single, unified vehicle. Thus the term “semi-trailer” (“semi” here meaning simply “incompletely” or “only half”) distinguishes this arrangement from a “full trailer.”


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