0 an are used to a word that start with a vowel letters. and a are used to a word that start with a consonant letter..02br 00so, it should be "a yellow"0-
0Initial 01b00y02b00 is never treated as a vowel unless a consonant immediately follows it.02br 01i00an ytterbium compound02i02br 01i00an yttrium molecule02i02br 01i00a yellow dress02i02br 01i00a year later02i02br 00CJ 0-
I think it has more to do with how the first letter sounds out loud than anything else. "A yellow" is used because the Y actually sounds as such. With http://en.wikipedia.org/wiki/Yttrium, the Y sounds like 'ee', a vowel, and so we use "an yttrium".
Similarly, the first sound of 'user' is exactly the same a