English is my native language, so I know that these combinations sound correct to me and other native speakers, at least in the U.S., but why? Why don't we consistently use or not use "of" for both in and out? Why are "out stock" and "in of stock" wrong?

Other examples with the same principle:

  • in jail and out of jail
  • in service and out of service
  • in time and out of time

I tried searching this question on this site and search engines, but didn't find an answer.

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    My belief (and it is an unsupported opinion so I am presenting it as a comment rather than an answer) is that the language has developed so that 'out of' is the opposite of 'in' and that there is no particular 'reason' why that happened. It is interesting to note that something similar happens in French ('hors de' is the normal opposite of 'dans' so far as I know) but not in German. Perhaps 'out of' merely followed the French convention as a result of the Norman Conquest. There are dialects where the 'of' is sometimes dropped as in "I got it out me pocket" but these are non-standard English.
    – BoldBen
    Commented Nov 14, 2021 at 2:01
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    I think the only answer here is: Because. Commented Nov 14, 2021 at 2:06
  • I haven't researched this, but doesn't British English observe a somewhat similar split in wording between "indoors" and "out of doors"?
    – Sven Yargs
    Commented Nov 15, 2021 at 18:33

2 Answers 2


It's because "in" is a preposition, while "out" is not*. In your examples, "out" is being used as an adjective or an adverb.

To have a true opposite for the phrase "in X", we need another prepositional phrase. Some languages have a single word for "out of" (the example that comes to mind right now is Latin, where "ex" means "out of"). But in English, we don't have a single-word opposite for "in". Sometimes we juxtapose the words "in" and "out" and treat them as opposites, but this is usually when they are directional adverbs (go in the door, go out the door) rather than prepositional states of being (in the room, out of the room).

So we say "out of X" simply because we would never* say "out X" and we don't have a word for "ex X" like in Latin.

Perhaps if you are craving some extra parallelism in your prepositional phrases, you could start using "into" as an opposite for "out of". While "into" has a slightly different meaning than "in", it could make for some semantically interesting examples!

  • into jail (upon initial incarceration)
  • into service (immediately after successful repairs?)
  • into time (...??)

EDIT: I forgot to mention, in many (most?) cases, "out" is usually analyzed as an adverb. "We ran out of milk." could be abbreviated as "We ran out." This shows that "out" is describing the verb "ran", instead of being inherently tied to the preposition "of".

*Please don't misunderstand what I'm saying! It is in fact possible to use "out" as a standalone preposition, meaning something like "through". Using "out" as a preposition used to be more common than it is today. In 1830, Alfred, Lord Tennyson wrote in "Adeline": "Thy roselips and full blue eyes / Take the heart from out my breast."

Nowadays the use of "out" as a preposition is more restrictive to certain phrases, for example "walk out the door" or "look out the window", and almost exclusively means "through". In contrast, today we would say "take the heart from out of my breast".

  • It seems likely that the single English preposition is outside (which doesn't take of, at least in British English).
    – Andrew Leach
    Commented Nov 14, 2021 at 10:17
  • *It's because "in" is a preposition, while "out" is not. * You should say what "out" is.
    – Greybeard
    Commented Nov 14, 2021 at 10:19
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    "Out" is an adverb. "In" can also be used as an adverb, but in the phrases from the questioner, "in" is being used as a preposition.
    – olkomat
    Commented Nov 14, 2021 at 10:21
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    They're both prepositions, but they have different affordances. As @YourGrammarGuide points out, the usual object of out is a path, not a container. In defines containers and gets used in container metaphors (like this one) everywhere. Out is a different matter and applies more often to movement than to position; if you want to implicate a container you need another preposition. Of course this is not logical, but it's the way things work now. Commented Nov 14, 2021 at 22:24
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    @JohnLawler You make a good point. The line between semantics and syntax is not always as clear as we might like. Your analysis is certainly reasonable. Words are words, and while we can try to put words into different categories, at the end of the day the "truth" of language lies in the unspoken competency of native speakers.
    – olkomat
    Commented Nov 14, 2021 at 22:40

The preposition "out" usually means "out through" (e.g.: "He went out the back door."). Therefore, you can't say "That product is out stock." On the other hand, "in" is a very common preposition with a variety of meanings, such as in the phrase "in stock." However, I don't know exactly why those prepositions developed as they did. BoldBen's suggestion (that they did so due to the influence of a foreign language, like French) is interesting.

  • +1 Right on. Look at the usages and constructions, not easy parallels. Commented Nov 14, 2021 at 22:25
  • "All kinds of interesting questions which the science knowledge only adds to the excitement, the mystery and the awe of a flower. It only adds. I don’t understand how it subtracts." Richard Feynman Commented Nov 15, 2021 at 1:45

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