Chambers 21st Century Dictionary Chambers Thesaurus Chambers Biographical Dictionary 1997 edition
Search results for 'AN':
a1 (used before a consonant or consonant sound, eg a boy, a one) or (used before a vowel or vowel sound, eg an egg, an hour)
An hour, we don't pronounce the h so the word sounds like it starts with a vowel. An for word beginning vowel, an office, an aeroplane, a for a word beginning with a consonant a dog, a doctor
An hour; an hotel; an historic; an heir.
A house; a hero; a helper; a hotspot.
It mainly depends on the first syllable. Where the h is definitely spoken then it is preceded with 'a'. Where the h is silent or the second syllable is the stressed part of the word then it is 'an'.
The word 'hour' is a good example. We don't say 'how-er', we say 'ours'.