Discussion Forum—A Way with Words, a fun radio show and podcast about language

First, welcome.

If I understand your question correctly, it has to do with why the form of the article a or an changes when a phrase is abbreviated. This is a very good question. While the answer is, in some ways, simple, there are some complications.

Importantly, for a and an, the choice has nothing to do with spelling and everything to do with sound. This holds true even if the words are never intended to be read aloud or spoken. If the sound following is a vowel sound, we choose an. If the sound following is a consonant sound, we choose a.

Since the letter H at the beginning of English words is sometimes pronounced as a consonant, and sometimes silent, we can demonstrate that the choice rests completely on the sound of the word, rather than on the spelling: a home; an honor; a horse; an hour. Like H, some words beginning with the letter U are pronounced with a consonantal /y/ sound at the beginning, while others are not: a uniform; an unknown; a unit; an underling.

Many consonant letter names start with vowel sounds (F, H, L, M, N, R, S, X). Conversely, one vowel name starts with a consonant sound (U). So abbreviations could change the form of the article from the full phrase:
a registered nurse / an RN
a medical doctor / an MD
a federal agent / an FBI agent

Here is where it can get complicated. Sometimes it is not clear if an abbreviation will be spelled out, or pronounced as a word:
an unidentified flying object / a UFO (Here it doesn't matter if you spell it /yoo-eff-oh/ or pronounce it /yoofoh/.)

But consider the case of the technology abbreviation SQL. It is sometimes spelled out /ess-kyoo-ell/, and sometimes pronounced /seequel/. You can tell which the author intends by looking at the article he or she chooses:
an SQL query
a SQL server

Likewise, the baseball abbreviation RBI is sometimes spelled out /ar-bee-eye/, and sometimes pronounced /ribee/.
an RBI
a RBI single (note that the photo caption has a RBI single, whereas the text of the article has an RBI double and an RBI single.)

My best advice is to trust your ear.

I also use a historic …, but smart people disagree. I can understand the disagreement on the basis of pronunciation. Some people pronounce the H while others don't, just as with an herb / a herb or an humble … / a humble … . (I pronounce the h of historic and humble, but not of herb.)

What's really odd about historic is that some people who pronounce the H still write and try to pronounce the article as an. I consider that practice an affectation.

[edited for format and some clarity]

"In New England ouah 'ah's disappeah; to wheah, I have no ideerrrr."

Someone told me that couplet years ago; apparently I never forgot it.   But JFK was a New Englander; adding 'r's to the end of certain words is not, there, a matter of elision (as far as I know) as they do in England, but just a regional deficiency idiosyncracy.   Or so I always believed until Bill 5 cited the opposite observation about Cuber.   I'd never heard that one, but it does argue for the same sort of thing going on as in England.

I can't absolutely disprove your accusation of affectation, Glenn, but it did occur to me a few years ago that while I pronounce "historic", "hypothesis" and so forth with a good strong 'h' sound at the beginning of a sentence, after a word ending in a vowel sound an initial unemphasized 'h' can get kind of lost.   So here's an alternate theory:   No one attempts to put "an" in front of an h-word whose first syllable is emphasized—no one, for example, writes or says "an hindrance" or "an harlequin".   But "an historic occasion" and "an hypothesis" might make some sort of sense.   I've experimented with it, and although I haven't adopted it fully I find it comes off the tongue alright.

Then again, maybe I'm just a showoff with an affectation .