A useful site: http://www.wsu.edu/~brians/errors/a.html

I agree with Cherry. Use "an" before unsounded h. an honorable peace. an honest error. See that when u make the same sound as the y in you, or o make the same sound as w in won, then a is used. a union a united front a unicorn a used napkin a U.S. ship a one-legged man

Hello, An honors degree The indefinite article 'a' become 'an' when preceding nouns (words) starting with the vowels ( u ,a ,e,o,i ) and also when preceding nouns starting with (h) ,where the 'h' is not really pronounced as in the word 'honor'. It is rather pronounced 'Onor', so it starts with an 'o' sound. Notice: an honor but a house ( since the 'h' as a consonant is pronounced in this word ).