# Why are 18-wheelers called "semi's"? **URL:** \ **Category:** Factual Questions\ **Created:** [August 11, 2008, 5:40pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630 "2008-08-11T17:40:15Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Broken\_Hoe](https://avatars.discourse-cdn.com/v4/letter/b/c57346/32.png) [@Broken\_Hoe](https://boards.straightdope.com/u/Broken_Hoe)\ **Post date:** [August 11, 2008, 5:40pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/1 "2008-08-11T17:40:15Z")
Or, just as often, “tractor-trailers”? A tractor **trailer** is a trailer associated with a tractor. But there is no tractor, there is a truck, and most of the time, there is a trailer attached. In this article about the bay bridge crash, the washington post can’t get it straight either. In one photo caption they call it a “semi” and in the other two, it’s a “tractor-trailer”. What gives with these names? What is the correct name for this vehicle? ---
**Author:** ![Argent\_Towers](https://avatars.discourse-cdn.com/v4/letter/a/e8c25b/32.png) [@Argent\_Towers](https://boards.straightdope.com/u/Argent_Towers)\ **Post date:** [August 11, 2008, 5:45pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/2 "2008-08-11T17:45:06Z")
The truck is the tractor. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [August 11, 2008, 5:48pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/3 "2008-08-11T17:48:32Z")
And the trailer is called a semi-trailer because it lacks a front axle, instead resting on the tractor. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 11, 2008, 5:49pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/4 "2008-08-11T17:49:05Z")
It’s short for ‘semi-trailer freighter’. A full trailer has wheels under both ends and is pulled by a drawbar. A semi-trailer has wheels under one end; the other end rests on another part of the vehicle. ---
**Author:** ![Broken\_Hoe](https://avatars.discourse-cdn.com/v4/letter/b/c57346/32.png) [@Broken\_Hoe](https://boards.straightdope.com/u/Broken_Hoe)\ **Post date:** [August 11, 2008, 5:54pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/5 "2008-08-11T17:54:38Z")
[QUOTE=yabob] And the trailer is called a semi-trailer because it lacks a front axle, instead resting on the tractor. [/QUOTE] Okay, but why is the whole assembly called “a semi”? As in, “we were being tailgated by a semi the whole way here”. A trailer can’t tailgate anyone, it can’t even move on its own. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [August 11, 2008, 5:59pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/6 "2008-08-11T17:59:22Z")
[QUOTE=Broken Hoe] Okay, but why is the whole assembly called “a semi”? As in, “we were being tailgated by a semi the whole way here”. A trailer can’t tailgate anyone, it can’t even move on its own. [/QUOTE] Because, as noted, it’s a natural shorthand for a more descriptive term. Saying “we were being tailgated by a semi-trailer freighter” gets a bit unwieldy. ETA: And what you are probably thinking of as a “tractor” is more properly called a “farm tractor”. The thing hauling the semi-trailer is a “road tractor”. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [August 11, 2008, 6:07pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/7 "2008-08-11T18:07:22Z")
[QUOTE=Broken Hoe] Okay, but why is the whole assembly called “a semi”? [/QUOTE] Same reason that you cook food in a microwave, some pianists play uprights, and people fly in jets. American English hates a long phrase if a short phrase works just as well. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [August 11, 2008, 6:50pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/8 "2008-08-11T18:50:02Z")
A tractor is a device principally used for pulling something else. The most common use of it is as shorthand for “agricultural tractor”, which pulls plows, harrows, reapers, combines, balers, wagons, and other such farm machinery. The large engine-and-cab unit that provides the motive power and steering to a semi is also a tractor, normally only called that in popular parlance in the combination “tractor-trailer”. Captain Kirk of Star Trek was wont to order things to be brought to his ship by tractor beam – the device emitting the tractor beam would no doubt be another form of tractor, an ironic twist when according to the ST backstory, Kirk was from Iowa. A semitrailer has wheels only beneath the rear – normally two axles bearing two tires at either end each. The front of the semitrailer rests on the large “fifth wheel” device at the back of the tractor, being held there by a giant pin and an assortment of other connections, the whole thing being supported by the two four-tire axles beneath the rear of the tractor unit providing motive power to the whole combination. Tandem (double) and triple units occasionally seen on major highways consist of, from the front, tractor unit, semitrailer, and one or two full trailers with axles and tires beneath both front and rear. ---
**Author:** ![ParentalAdvisory](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)\ **Post date:** [August 11, 2008, 7:10pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/9 "2008-08-11T19:10:34Z")
Thanks for this thread. I always wondered where the tractor part was coming from with discussions of semi’s. It still doesn’t make sense why they’re called tractors, but at least I’ll know what people are talking about. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 11, 2008, 7:16pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/10 "2008-08-11T19:16:18Z")
[QUOTE=ParentalAdvisory] It still doesn’t make sense why they’re called tractors… [/QUOTE] A tractor is a machine that pulls other equipment. A farm tractor pulls farm equipment. A road tractor pulls trailers or semitrailers. Your car or truck is a tractor unit if it’s pulling a trailer, presumably. I suppose a locomotive could be called a tractor unit, although they are quite capable of pushing as well, since railway cars are guided by the rails in a way that road trailers aren’t. ---
**Author:** ![Mister\_Rik](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mister_rik/32/491_2.png) [@Mister\_Rik](https://boards.straightdope.com/u/Mister_Rik)\ **Post date:** [August 11, 2008, 7:17pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/11 "2008-08-11T19:17:51Z")
[QUOTE=CookingWithGas] Same reason that you cook food in a microwave, some pianists play uprights, and people fly in jets. American English hates a long phrase if a short phrase works just as well. [/QUOTE] With the exception of faux-intellectuals who use “at the present time” when “now” would work better. 😃 > [@Polycarp](#): > > A tractor is a device principally used for pulling something else. And furthermore: attract - pull/draw toward retract - pull/draw back detract - pull/draw apart contract - pull/draw together extract - pull/draw out distract - pull/draw attention protract - pull/draw out longer ---
**Author:** ![Dinsdale](https://avatars.discourse-cdn.com/v4/letter/d/97f17d/32.png) [@Dinsdale](https://boards.straightdope.com/u/Dinsdale)\ **Post date:** [August 11, 2008, 7:34pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/12 "2008-08-11T19:34:36Z")
If you don’t mind a bit of a highjack, on a trip a week ago I noticed that not a single semi was a cab-over. I assume this change was done for fuel efficiency reasons, but had never noticed it before. Saw hundreds of trucks, and not one was cab-over. Anyone got any info on when this conversion occurred? I apparently was asleep at the wheel… ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [August 11, 2008, 7:41pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/13 "2008-08-11T19:41:35Z")
Perhaps you would find the British “articulated lorry” less confusing? Except that they sometimes abbreviate that to “arctic”, which sounds like they are referring to a “reefer”, which, in turn, has its own problems with connotation. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [August 11, 2008, 7:41pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/14 "2008-08-11T19:41:44Z")
[QUOTE=Polycarp] Tandem (double) and triple units occasionally seen on major highways consist of, from the front, tractor unit, semitrailer, and one or two full trailers with axles and tires beneath both front and rear. [/QUOTE] Nitpick: Quite commonly double and triple trailers use 5th wheel attachments at each point, making each unit a semitrailer. This configuration is known (in these parts, at least) as a Super B. See, for example, this trailer manufacturer’s [grain trailers](http://www.doepker.com/agriculture.html), specifically the 3rd and 4th items down. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 11, 2008, 7:48pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/15 "2008-08-11T19:48:45Z")
[QUOTE=yabob] Perhaps you would find the British “articulated lorry” less confusing? Except that they sometimes abbreviate that to “arctic”… [/QUOTE] I only see ‘artic’ used around here with respect to buses. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [August 11, 2008, 7:51pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/16 "2008-08-11T19:51:16Z")
[QUOTE=yabob] Perhaps you would find the British “articulated lorry” less confusing? Except that they sometimes abbreviate that to “arctic”, which sounds like they are referring to a “reefer”, which, in turn, has its own problems with connotation. [/QUOTE] “artic”. “Tractor” is also the term for an airscrew that pulls the plane forward; originally a “propellor” was what we now call a “pusher”, though that usage has been defunct for a long time - possibly since a wartime request for fifty air_crew_ to be sent to a station that was fully staffed but lacked air_screws_. :smack: ---
**Author:** ![brewha](https://avatars.discourse-cdn.com/v4/letter/b/91b2a8/32.png) [@brewha](https://boards.straightdope.com/u/brewha)\ **Post date:** [August 11, 2008, 7:55pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/17 "2008-08-11T19:55:14Z")
[QUOTE=Dinsdale] If you don’t mind a bit of a highjack, on a trip a week ago I noticed that not a single semi was a cab-over. I assume this change was done for fuel efficiency reasons, but had never noticed it before. Saw hundreds of trucks, and not one was cab-over. Anyone got any info on when this conversion occurred? I apparently was asleep at the wheel… [/QUOTE] There used to be a length limit on the tractor/trailer combo. By shortening the tractor, you could have a longer trailer and haul more cargo. You could accomplish this by putting the cab over the engine instead of behind it. I believe the laws have been changed to limit the length of the trailer only. Since cab-overs are no longer neccesary, they aren’t gettting built or used much anymore. ETA From Wiki: > [@](#): > > Three years later, the Surface Highway Transportation Assistance Act of 1982 relaxed weight and length standards and imposed a new excise tax on heavy trucks and the tires that they use. No longer was the overall length of semi-trailer combinations restricted; rather, only the trailer was specified, to be not greater than 53’ in length. Individual states retained more restrictive overall length laws, but fundamentally, the rules had changed forever. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [August 11, 2008, 7:56pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/18 "2008-08-11T19:56:47Z")
[QUOTE=Sunspace] I only see ‘artic’ used around here with respect to buses. [/QUOTE] Well, OK, it’s YOUR turf, but here’s a couple examples: > **[Tyranni.com is for sale | HugeDomains](https://www.hugedomains.com/domain_profile.cfm?d=tyranni.com)** > > The leading source brandable domain names. Competitive prices. Excellent service. Get your domain name today. [http://www.manbymotorplex.com/res\_website.asp?supplierCode=MBY100&page=artic](http://www.manbymotorplex.com/res_website.asp?supplierCode=MBY100&page=artic) ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [August 11, 2008, 8:20pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/19 "2008-08-11T20:20:09Z")
[SDSTAFF Jeff speaks.](http://www.straightdope.com/mailbag/msemi.html) ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [August 11, 2008, 8:49pm UTC](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630/20 "2008-08-11T20:49:08Z")
[QUOTE=brewha] Since cab-overs are no longer neccesary, they aren’t gettting built or used much anymore. [/quote] Wow. Future generations will have _no idea_ what C.W. McCall was talking about when he spoke of a “cab-over Pete”. [Next page](https://boards.straightdope.com/t/why-are-18-wheelers-called-semis/459630.md?page=2)