Use an before a word that starts with a vowel sound. If it does not start with a vowel sound, use a. For example:|
Using A and AnThere is sometimes confusion about whether to use an or a, particularly with abbreviations. (The words an and a are known as articles.)
The sound of a word's first letter determines which to use. If the word starts with a vowel sound, you should use an. If it starts with a consonant sound, you should use a.
(The abbreviation RTA starts with an a sound (i.e., ar-tee-ay.)
Click on an or a:
BEWARE OF ABBREVIATIONS
Abbreviations that start with the consonants F, H, L, M, N, R, S, and X attract an because they start with vowel sounds. For example:
Abbreviations that start with the vowel U attract a because U starts with the consonant sound y. For example:
An acronym is an abbreviation that is spoken like a word, e.g., BUPA, FOD, FEDEX. Therefore, as the first sound of FEDEX is f, use a and not an.
AN HISTORICAL OR A HISTORICAL?
Letters and sounds do not always correlate in English.
When pronouncing the words historic and historical, the accent falls on the second syllable, and many pronounce them as starting with a vowel. For those people, it is appropriate to use an before historic and historical. Therefore, you have a choice depending on what sounds best for you.
There is a lot of leniency on this issue. If you're still unsure, opt for a historical and a historic as these remain preferable – especially in formal writing.
Read about the difference between historic and historical.