You use `an` in front of an acronym when the first letter of it has a vowel _sound_, even if the letter itself is not a vowel. Example: "A member of parliament

You use `an` in front of an acronym when the first letter of it has a vowel _sound_, even if the letter itself is not a vowel. Example: "A member of parliament talked to me." "An MP talked to me." Here when pronouncing the m in `MP` the vowel sound for e is made, as in 'em'.