# Tractor-Trailer = Semi: Semi-what? **URL:** \ **Category:** Factual Questions\ **Created:** [November 2, 2002, 1:37pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996 "2002-11-02T13:37:43Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [November 2, 2002, 1:37pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/1 "2002-11-02T13:37:43Z")
Regarding tractor-trailers, to what does the term “semi” refer? - Jinx ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [November 2, 2002, 1:42pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/2 "2002-11-02T13:42:41Z")
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. ---
**Author:** ![everton](https://avatars.discourse-cdn.com/v4/letter/e/ecb155/32.png) [@everton](https://boards.straightdope.com/u/everton)\ **Post date:** [November 2, 2002, 1:47pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/3 "2002-11-02T13:47:37Z")
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. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [November 2, 2002, 3:01pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/4 "2002-11-02T15:01:40Z")
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. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [November 2, 2002, 4:05pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/5 "2002-11-02T16:05:16Z")
Yep, 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. ---
**Author:** ![Anachronism](https://avatars.discourse-cdn.com/v4/letter/a/e56c9b/32.png) [@Anachronism](https://boards.straightdope.com/u/Anachronism)\ **Post date:** [November 3, 2002, 6:17pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/6 "2002-11-03T18:17:52Z")
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’. ---
**Author:** ![buckgully](https://avatars.discourse-cdn.com/v4/letter/b/eb9ed0/32.png) [@buckgully](https://boards.straightdope.com/u/buckgully)\ **Post date:** [November 3, 2002, 7:44pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/7 "2002-11-03T19:44:38Z")
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”? ---
**Author:** ![Anachronism](https://avatars.discourse-cdn.com/v4/letter/a/e56c9b/32.png) [@Anachronism](https://boards.straightdope.com/u/Anachronism)\ **Post date:** [November 3, 2002, 8:54pm UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/8 "2002-11-03T20:54:19Z")
> [@](#): > > \*Originally posted by buckgully \* > \*\*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’. ---
**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\ **Post date:** [November 4, 2002, 2:08am UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/9 "2002-11-04T02:08:36Z")
Staff Report: [Why are tractor-trailer rigs called “semis”?](http://www.straightdope.com/mailbag/msemi.html) ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [November 4, 2002, 2:47am UTC](https://boards.straightdope.com/t/tractor-trailer-semi-semi-what/134996/10 "2002-11-04T02:47:22Z")
From Merriam-Webster’s online dictionary: > [@](#): > > Main Entry: semi·trail·er > Pronunciation: 'se-"mI-"trA-l&r, 'se-mE-, -mi- > Function: noun > Date: 1919 > 1 : a freight trailer that when attached is supported at its forward end by the fifth wheel device of the truck tractor > 2 : a trucking rig made up of a tractor and a semitrailer