Possible Duplicate:
When should I use “a” vs “an”?
Do you use “a” or “an” before acronyms?
Consider the following sentence:
"This is a one-time deal" sounds right
"This is an one-time deal" sounds wrong
"One" is pronounced the same as "won", which wouldn't require an "an".
Is it proper/required to use the 'an' before a vowel rule when it just sounds wrong?