Like my advice/advise pet peeve, an & a bother me to bits. Most people seem to be unaware of the rule.
The rule being: Put 'a' before words that start with consonants and 'an' before words that start with vowels.
I'm unsure why people aren't able to follow this. But it irks me. It irks me to no end!
The thing is, it's a bit more complicated than that. It's not just words that start with a vowel, but ones that SOUND like they start with a vowel.
Here's my example:
It took me a hour to drive home.
EHHHHHHHHHHHHHHHHHHHHHH. (That's a buzzer sound)
This is wrong.
Hour sounds like 'our' and thus needs 'an'. Not 'a'.
Draw me a one dimensional sketch.
One sounds like 'won' and thus needs 'a' not 'an'.
Are you getting my drift?
The best thing to do is sound it out. Does it sound odd? Try it with the reverse and see if it sounds better. If you can't tell which one sounds better, I can't help you.
And no, I am not pedantic. You should see my friend Noelle. :-/