At first glance, the question of whether to us a or an may seem to be too simplistic to detail in a full article, but even if you are a native speaker of English, you probably follow more rules to get it right than you realize! Let's take a fun look at the twisted road we traverse to choose the right indefinite article.

Our starting point is here: if a word begins with a consonant, it takes the article a. If it begins with a vowel, it instead takes an.

Ex:
--a baseball; a dinner plate; a French chateau
--an acrobat; an overcoat; an ice-cold beer

Hang on, now; we're approaching a curve. Sometimes, pronunciation rather than spelling determines which article to use. If a word that begins with a vowel sounds like it begins with a consonant, it will require a regardless its spelling.

Ex:
a Utopian ideal; a usability study; a U-turn

Likewise, if the word, when said aloud, sounds like it starts with a vowel, it takes an even if its spelling begins with a consonant.

Ex:
an homage; an hour's wait; an honorary degree

The road seems to be straightening again, but don't let those last examples fool you into thinking that a silent letter at the beginning of a word always reverses the article used. In many cases, a silent first letter is a consonant but is followed by a pronounced consonant, resulting in using a just as in the simple rule.

Ex:
a psychological study; a gnu; a knife

There's a detour ahead. Some words, notably those that start with h, may have an initial silent letter depending on the region or dialect of the English speaker using them. For instance, a British speaker will often pronounce the h in herb, resulting in "a herb dressing." An American speaker looking at the same condiment likely will regard the h as silent and instead declare it "an herb dressing." In these cases, let the context of the writing rule. It would not be appropriate to "correct" the articles traditionally used in another dialect just because they are not the ones used in our own.

Ex: Either article can be correct.
--a historic document OR an historic document
--a humble attitude OR an humble attitude

Now tighten your seatbelt for a hairpin turn. When letter combinations are used as pseudo-words, the pronunciation rule rather than spelling rule dominates. Combinations that are pronounced just as though they are ordinary words (acronyms) follow the basic rule of consonant=a or vowel=an. However, combinations that are orally spelled out rather than pronounced as words (initialisms) take the article that fits the pronunciation of the name of the letter they begin with. This is not to be confused with the sound that letter makes, but the name of the letter itself. Initialisms are the trickier combinations, regularly confusing even native English speakers.

Ex: acronyms
--a UNICEF fund drive (UNICEF is pronounced "yooniseff" thus taking the y-consonant article, a.)
--a SEAL training exercise (SEAL is pronounced "seel" thus taking the s article, a.) --a NATO goal (NATO is pronounced "naytoe" thus taking the n article, a.)

Ex: initialisms
--a CIA operative (CIA is spelled aloud, and C sounds like "see" thus taking the s article, a.)
--an EPA study (EPA is spelled aloud, and E sounds like "ee" thus taking the e article, an.)
--an FBI investigation (FBI is spelled aloud, and F sounds like "eff" thus taking the short-e or schwa article, an.)
--an SOS signal (SOS is spelled aloud, and S sounds like "ess" thus taking the short-e or schwa article, an.)

This rough ride makes it clear: the question of a versus an really isn't simple at all.

AnnaLisa Michalski writes and publishes the ezine Word-wise http://www.adminmaven.com/wordwise.htm She owns and operates Admin Maven, a virtual assisting service specializing in proofreading and copy editing. http://www.adminmaven.com/