An is used before words, abbreviations, acronyms, or letters that begin with a vowel sound, regardless of their spelling.
A vowel is most often identified as a letter that is not a consonant. More specifically, a vowel is a sound that when paired with a consonant makes a syllable.
A vowel is any sound that a letter makes that is not a consonant sound.
There are five English vowels,
A, E, I, O, U.
Sometimes, Y can also function as a vowel, but it is not considered a vowel in and of itself.
