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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.

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Here's an interesting article on the history of "I". –  tylerharms Dec 31 '12 at 0:06
    
@tylerharms That is interesting. Do you know of anything about O? –  ctype.h Dec 31 '12 at 0:47
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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. Dec 31 '12 at 1:15
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@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 Dec 31 '12 at 1:21
    
This is more of a hunch. I think O is capitalized to avoid confusion with 0 (zero). –  Noah Dec 31 '12 at 10:39
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2 Answers

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/.

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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 Dec 31 '12 at 13:23
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