use of "a" or "an"

[babing20]

i guess this is a very simple question but i always get confused with it. Which article should be used for abbreviations. example:

a Return Merchandise Authorization
a RMA

or should it be an RMA? help!

[Joe Carillo]

The article to use will depend on how the first syllable of the word, phrase, or abbreviation is pronounced--"a" when the first letter or first syllable is pronounced as a consonant sound, as in "a Return Merchandise Authorization" or "a PhD in linguistics," and "an" when the first letter or first syllable is pronounced as a vowel sound, as in "an RMA" or--to use another set of examples--"an X" and "an extemporaneous speech."