a or an

It should "a" one hour meeting. You use "a" when the word immediately after the indefinite article is pronounced as though it begins with a consonant, and you use "an" when the word immediately after the indefinite article is pronounced as though it begins with a vowel. Therefore, you say "a" one hour meeting because one is pronounced as though it begins with a "w"--one is pronounced the same as won. You say "a" useful idea because useful is pronounced as though it begins with a "y." You would say "an" hourly occurrence because hourly is pronounced as though it begins with a vowel, despite the "h" at the beginning of the word. Is this clear to you?

Thanks, Setanta, it is very clear.