Semi trucks are known by many other monikers, from tractor-trailer and 18-wheeler to the more informal big rig. Some people even drop the truck and just call them semis. Considering how much larger these vehicles are than most other trucks on the road, a title that essentially means half truck or partial truck may seem a little misleading.
But as International Used Truck Centers explains, semi truck is actually a shortened version of semi-trailer truck (or sometimes semi-tractor-trailer). Basically, a regular trailer can typically bear its own weight and gets attached to the vehicle in front of it with a drawbar or something comparable. A semi-trailer, on the other hand, is a type of trailer that only has rear wheels; its wheel-less front is hitched atop the back wheels of a tractor, which then bears some of its weight.
There’s a lot of variation when it comes to describing and defining these vehicles. Sometimes, people call the tractor itself a semi-truck or semi-tractor, presumably because it’s one of two parts needed to make a full semi-tractor-trailer. It could also just be for clarity’s sake, since truck could mean a pick-up truck (or some other type of truck), and tractor could refer to a farm tractor (or some other type of tractor).
In casual conversation, if someone mentions a semi or a semi truck—maybe to wonder why they often have spikes on their wheels—there’s a good chance they’re talking about the whole big rig, rather than just the front or back. But if the conversation includes truck drivers (who’d probably be familiar with the technical differences between all the terms), you might want to ask for clarification.
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