Rule for using the word 'an'

As a general rule:
AN is used when the next word begins with a vowel......an Apple.
A is used when the next word begins with a vowel.........a Book.
BUT.....
We use AN before a vowel sound, even if it is written with a consonant...
e.g. An hour, an honour,
We use A before a consonant sound, even if it is written with a vowel....
e.g. A university, a one-dollar n

Woordward - Clear and concise!

I would say we use "a/an" before a "semi-consonant vowel sound "-for instance: a University, an hour.

Possible.
With... Uncle, underwear, udder.....you need to use AN.
I have an uncle that has a picture of an udder on his underwear.
(How's that for a totally bizarre sentence?)
Those don't have a semi-consonant vowel sound.
So the question is.......
How do we know which letters have a semi-consonant vowel sound and when?

/ju:/ sound, /awr/. It's difficult to explain without being able to transcribe.

Check these two threads:

What about the word "hotel"? I use an, is that correct?

AnonymousWhat about the word "hotel"? I use an, is that correct?

No. The H in hotel is pronounced. Use "a."
Did you realize you had posted on a thread that was from the year 2003?

Please help...? Does the general rule also apply to abbreviations?

It's an honor.

Why do some people say, "an historic," when the "h" is not silent?