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When should I use “a” vs “an”?

While I was reading a book, I faced the following sentence:

There is a one-to-one correspondence between the two sets of quantities.

So, my question is: why the indefinite article "a" is not "an" in this phrase? The following word starts with a vowel, so shouldn't it be "an"?

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marked as duplicate by Marthaª, Mr. Shiny and New 安宇, z7sg Ѫ, Matt Эллен, JSBձոգչ Nov 10 '11 at 16:26

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Also semi-relevant: english.stackexchange.com/questions/2429/… –  Marthaª Nov 10 '11 at 16:10
    
You could solve your problem by saying "bijective" instead... –  dmckee Nov 11 '11 at 0:08

2 Answers 2

The word one is pronounced as beginning with a /w/ sound. Because this is a consonant sound and not a vowel sound, the indefinite article a is used instead of an.

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People, this is such an obvious duplicate that you have no excuse for posting answers to it. –  Marthaª Nov 10 '11 at 16:19

It's not about spelling but about the pronunciation. 'one' is spelled with a vowel 'o' at the beginning, but it is the sound 'w' (a bilabial glide) that is not really a vowel.

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People, this is such an obvious duplicate that you have no excuse for posting answers to it. –  Marthaª Nov 10 '11 at 16:19
    
@Marthaª: I went to check the other answer first (I was going to immediately vote to close as duplicate), but I didn't see in the other question anything addressing this particular case. 'o' certainly looks like a vowel to most people (naively), so why doesn't the rule apply? –  Mitch Nov 10 '11 at 17:47
    
It's still the same rule: if it sounds like a consonant, use 'a'. We don't need to have separate questions about every letter of the alphabet. –  Marthaª Nov 10 '11 at 18:02
    
@Marthaª: What about “a” or “an” for words that don't start with vowels but sound like they're starting with a vowel? That's a reasonable question if you don't know English and pronounce initial "st" as /est/. I see similar reasonableness with "one". It's a seeming exception to the rule that needs explanation. –  Mitch Nov 10 '11 at 18:27

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