Tractor-Trailer = Semi: Semi-what?

Regarding tractor-trailers, to what does the term “semi” refer?

  • Jinx

Ever see a full-sized truck pulling a trailer as big as the truck itself? That’s a full trailer. Semi-trailer means the tractor carries no load, just the trailer.

As I understand it the semi is the back part of the vehicle and the tractor is the front part, so “semi” is the “back half”. I could be wrong, though - we don’t call them that over here.

My understanding is a that a trailer is a towed vehicle having wheels at or near all four corners. A semi-trailer has wheels only at the back end with the front end supported by the towing vehicle. The Merriam-Webster Collegiate Dictionary agrees with this definition.


As David says, a full trailer would have most of its weight supported on it’s own wheels.

A Semi trailer has a great deal of the weight supported by the tractor.

Just to clarify what David and enipla said. A Semi trailer needs the towing vehicle to support a large portion of the load, in the case of a ‘Tractor Semi-trailer combination vehicle’ (see why it is shortened to ‘Semi’) usually about half. A semi trailer can be converted to a full trailer by adding a dolly under the front.This is what you see when you see a truck hauling ‘doubles’, the first one is a semitrailer and the second is a full trailer.

A full trailer does not need to have wheels at all four corners, it can have one axel in the middle that supports the majority of the weight. A full trailer with an axle in the middle is supposed to be loaded so the hitch only supports a small amount of the weight. So a small trailer that is pulled behind a car is a ‘full trailer’ but the big trailer pulled by an 18 wheeler is only a ‘semi-trailer’.

I may be completely wrong here as I currently have a cold and I just took some NyQuil, but isn’t “semi” short for “semi-articulated”?

As it refers to trucks ‘semi’ is short for ‘tractor semi-trailer’ or more accurately ‘tractor semi-trailer combination vehicle’.

Staff Report: Why are tractor-trailer rigs called “semis”?

From Merriam-Webster’s online dictionary: