Someday, I need to look up how the actual rule for use of "a" vs. "an" is written. Most people, including me, have always said that "an" is used in front of a vowel. "This is an unusual task." But this is not always true. How about, "this is a useful item." In this case, I suppose we use "a" because despite starting with a vowel, "useful" really starts with a "y" consonant sound, as in "you."