use of a or an

[dwilkinson]

Can you tell me when you use a or an?
My boss used "a" in front of the abbreviation HR( Human Resources) in a memo and I said it should be "an". Comments?

[Casiopea]

Welcome, dwilkinson.

The rule is about pronunciation--sound:

"a" before consonant sounds
e.g., a USA tape, [ju:]
(the sound [j] = y, a glide, a non-vowel)

The letters <U, u> (in USA) look like vowels but they are pronounced with a consonant sound, [j] as in yellow.

"an" before vowel sounds
e.g., an HR memo [ei]
(the sound [ei] as in hay, a vowel sound)

The letters <H, h> (in HR) look like consonants but they are pronounced with a vowel sound, [ei] as in hay.

Does that help?