A note for Americans regarding British grammar.
In British usage it is both acceptable and common to use 'an' before many words beginning with 'h' or before acronyms, in addition to words beginning with vowels. So 'an hour' is perfectly correct and is spoken as a 'liaison'. There are exceptions, it can often boil-down to what sounds better. 'A hour' would usually also be considered correct, but a bit ugly-sounding.
American rules are more proscriptive and insist that 'an' is only used in front of words beginning with a vowel.
British usage appears to stem from our close association with French where 'h' is silent.
"an hour" is fine in the US as the "h" is silent everywhere, to the best of my knowledge, and the a/an rule usually goes by pronunciation, not spelling (thus "a university" since that starts with a consonant sound but "an honour" since that starts with a vowel sound).
But your comment applies to things such "an hotel" or "an historic occasion" where I pronounce the "h", yet the article "an" is nevertheless possible in conservative British English.
Thanks. My mistake, I clearly mis-remembered. A check of the OED shows you are right.
I wondered about also mentioning the reverse for some/many words beginning with 'u' but thought it might confuse things.
Yes, 'hotel' and 'historic' are classic examples.