There are three single-letter words. They are the article a, the pronoun I, and the interjection O. The pronoun I and the interjection O are always capitalized, but the article a follows normal capitalization rules. Why is this the case? When and where did this originate?

Edit: As described in the answers to Question 7988, the pronoun I was first used in the 13th century, to avoid confusion with the dotless j, and was retained as a typographic convention because i "gets lost" due to its small size. However, this does not explain why O is always capitalized, but a is not, since a and o are approximately the same size.

  • 1
    Here's an interesting article on the history of "I".
    – tylerharms
    Commented Dec 31, 2012 at 0:06
  • @tylerharms That is interesting. Do you know of anything about O?
    – ctype.h
    Commented Dec 31, 2012 at 0:47
  • 4
    I can't say for sure (this only A speculative theory) – but I'd think this is partly because a is A rather common article, and using A capitalized letter with such A common but relatively insignificant word would be more of A distraction than A help in reading, particularly when the word might be found so many times in A single sentence.
    – J.R.
    Commented Dec 31, 2012 at 1:15
  • 3
    @ctype.h: I can only guess that it's to show emphasis. In the same way that J.R. has illustrated the lack of emphasis we want the letter a to have in a given sentence, we use the exhortation O only to create emphasis.
    – tylerharms
    Commented Dec 31, 2012 at 1:21
  • This is more of a hunch. I think O is capitalized to avoid confusion with 0 (zero).
    – Noah
    Commented Dec 31, 2012 at 10:39

3 Answers 3


English capitalization is a can of worms nestled inside the larger swamp of English spelling -- i.e, there is almost never any good reason for why anything is spelled, punctuated, or capitalized any particular way -- they just are. Sometimes.

However, one contributing reason for this particular feature is probably the fact that both I and O are usually stressed (as one can tell from the fact that they contain diphthongs, which require stress to avoid reduction), while a is never stressed and therefore always reduced to /ə/.


The difference between a and O is that O, as an interjection, represents a lightly more independent linguistic unit. You might say:

O! That’s a great plan.

but not

Hey, that's a...! Sorry, I just like indefinite articles.

Hence a has a more subtle effect that doesn’t modify the meaning substantially, but rather needs other parts of speech to complement it.

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    O is the vocative particle, O Abody97!
    – tchrist
    Commented Dec 31, 2012 at 13:23

Oh! is an interjection. (I disapprove of using O for Oh!) For me, O is not an interjection, but could be called a case marker for the vocative case; that is, a noun used in direct address. In "O John, come here," O goes with John and could be called a preposition, as prepositions substitute for terminations on nouns which are commonly considered as case markers. For example "John's" uses 's, a case ending for possessive case. But "of John" is equivalent, using a prepostion for the same function. O indicates direct address, but also is emphatic. Perhaps it was thought that using a capital for O indicated emphatic or "pay attention" for which function capital letters are sometimes used, with pedants claiming "That is shouting."

  • Hello, and welcome to the EL&U. Your answer could be much improved by formatting it and providing references. See tour.
    – fev
    Commented Aug 4, 2021 at 6:58
  • O is the older form of both interjection and vocative marker (see the OED). But languages do change, and these days, it is usual to distinguish "Oh" for the exclamation of surprise even if a lot of people do confuse it.
    – Stuart F
    Commented Aug 4, 2021 at 9:28

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