# What is the little door within the big door on a semi trailer for? **URL:** \ **Category:** Factual Questions\ **Created:** [June 30, 2017, 6:34pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220 "2017-06-30T18:34:37Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Mean\_Mr.Mustard](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mean_mr.mustard/32/3865_2.png) [@Mean\_Mr.Mustard](https://boards.straightdope.com/u/Mean_Mr.Mustard)\ **Post date:** [June 30, 2017, 6:34pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/1 "2017-06-30T18:34:37Z")
Google didn’t help me find the answer or even an image. I saw one yesterday and got to thinking; what on earth could this be used for? It is a small - maybe 12 inches high by 6 inches wide - door on the lower right corner of the right door on a semi trailer. It has sturdy hinges and a sturdy latch that looks like it will accommodate a padlock. I cannot figure out its purpose, but I’ll bet a Doper knows. mmm ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [June 30, 2017, 6:48pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/2 "2017-06-30T18:48:50Z")
Apparently, [the SDMB has been there and done that already.](http://boards.straightdope.com/sdmb/showthread.php?t=206992http://)😃 ---
**Author:** ![Procrustus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/procrustus/32/2994_2.png) [@Procrustus](https://boards.straightdope.com/u/Procrustus)\ **Post date:** [June 30, 2017, 6:50pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/3 "2017-06-30T18:50:20Z")
From the [internet](https://www.quora.com/Trucking-Why-is-there-a-small-rectangular-door-in-the-rear-door-of-refrigerated-Tractor-Trailers): > [@](#): > > The little door on the back allows him to visually inspect the load before he gets down the road without breaking the seal. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [June 30, 2017, 8:06pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/4 "2017-06-30T20:06:27Z")
Somebody post a photo? The Internet cit above is only to a latch for the whole shebang, I think. ---
**Author:** ![Mean\_Mr.Mustard](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mean_mr.mustard/32/3865_2.png) [@Mean\_Mr.Mustard](https://boards.straightdope.com/u/Mean_Mr.Mustard)\ **Post date:** [June 30, 2017, 8:07pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/5 "2017-06-30T20:07:53Z")
> [@Czarcasm](#): > > Apparently, [the SDMB has been there and done that already.](http://boards.straightdope.com/sdmb/showthread.php?t=206992http://)😃 Huh! Thanks. mmm ---
**Author:** ![Mean\_Mr.Mustard](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mean_mr.mustard/32/3865_2.png) [@Mean\_Mr.Mustard](https://boards.straightdope.com/u/Mean_Mr.Mustard)\ **Post date:** [June 30, 2017, 8:11pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/6 "2017-06-30T20:11:22Z")
> [@Leo\_Bloom](#): > > Somebody post a photo? The Internet cit above is only to a latch for the whole shebang, I think. Took a hot minute, but I found a [photo](http://www.utilitytrailer.com/images/trailers/features/f_dsc_4045.jpg). ETA: Zoom in for a good look. mmm ---
**Author:** ![Just\_Asking\_Questions](https://avatars.discourse-cdn.com/v4/letter/j/ba9def/32.png) [@Just\_Asking\_Questions](https://boards.straightdope.com/u/Just_Asking_Questions)\ **Post date:** [June 30, 2017, 9:19pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/7 "2017-06-30T21:19:16Z")
I’ve seen trucks going down the road with both the front access door and the back one open, so I figure at least sometimes, the drivers use the doors to get rid of that stale freezer smell. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [July 1, 2017, 12:04am UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/8 "2017-07-01T00:04:26Z")
Its to let the cat in, to chase the rats out. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [July 1, 2017, 2:42pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/9 "2017-07-01T14:42:35Z")
> [@Mean\_Mr.Mustard](#): > > Took a hot minute, but I found a [photo](http://www.utilitytrailer.com/images/trailers/features/f_dsc_4045.jpg). > > ETA: Zoom in for a good look. > mmm Thanks. I never saw (or noticed) one before. ---
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [July 1, 2017, 6:34pm UTC](https://boards.straightdope.com/t/what-is-the-little-door-within-the-big-door-on-a-semi-trailer-for/790220/10 "2017-07-01T18:34:12Z")
Usually on refrigerator truck trailers. If a visual look is needed, much better/efficient to open a small door than half of the whole end of the trailer. On my small long distance small boats the ice/refrigerator boxes will have a small door on top included in the main top hatch you can reach through and remove small items without dump a lot of cool air that needs to be cooled again. Some even add Styrofoam blocks as room opens up through use to reduce the amount of air needing to be re-cooled. Or so I have been told. On top would be better on the trailers also but the saving on the human in time & effort with it at a ground height is good nuff.