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A Staff Report from the Straight Dope Science Advisory Board

Why are tractor-trailer rigs called "semis"?

Dear Straight Dope:

I was on a road trip with some friends the last couple days, and we got to wondering why people call tractor trailer rigs semi trucks. Wouldn't a semi truck be only sort of a truck? I would consider an eighteen wheeler a full fledged truck. Can you enlighten me?

— Adam Sund

Dear Adam:

The full name of that thing sucking your doors off on the interstates is "tractor semitrailer." The part in the back is called a semitrailer because only the back end sits on wheels (and thus "trails"); the front end is supported by the tractor. A full trailer is fully supported by its wheels (commonly on axles at front and back, in the case of heavy over-the-road vehicles) and is connected to the tractor (or, far more commonly, the semi in front of it) by a drawbar. A truck need not have a trailer to be considered a truck, of course. Look at one of the trucks you can rent from U Haul or Ryder. No trailers there.

— Jeff

Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.

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