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I have a little doubt about use if article before the word open. Which one is correct in the following examples: There exists an open interval. There exists a open interval. What is the rule?

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The correct option is

There exists an open interval

Because open starts with a vowel.

  • What is the actual rule. We use A University, not an university, please clarify @CowperKettle – MKS Mar 9 '19 at 6:10
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First one is still the better one:

There exists an open interval

But i think you can should use:

There is an existing open interval

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    You mean the first one, surely, not the second. – Nathan Tuggy Mar 9 '19 at 6:20
  • @NathanTuggy Oh sorry, edited my answer, thanks for correcting my stupid mistake – U10-Forward Mar 9 '19 at 6:21
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Which to use depends on whether the following word starts with a vowel sound. "University" start with the y-consonant sound, so it's "a university", but "hour" starts with a vowel sound, so an "an hour". Interval also starts with a vowel sound, so it is "an interval".

If "university" were pronounced with an 'oo' sound, rather than a 'yoo' sound, it would use "an" as well.

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