Qfyilyi

The important point to remember is the following:

Written language is a representation of the spoken word.

Thus, the answer is "If the word following the indefinite article begins with a vowel sound, use an; if it begins with a consonant sound, use a."

In the case of initialisms and acronymns, use the exact rule above. For initialisms (e.g. "US"), the individual letters are pronounced. With what sound does the first pronounced letter begin? In the example "US", the first sound is /j/ (or "y"). This is a consonant sound, despite the letter "U" being a vowel; thus, you use a, as in a US dollar.

Contrast this with the initialism "RPM", which begins with the consonant "R" but is pronounced starting with /a/; thus, you use an, as in an RPM counter.

The rule about the usage of a and an as indefinite articles is that an is used before a vocal sound.

• A warranty (/ˈwɑːrənti/)


• A user (/ˈjuːzər/)


• A one-way (/ˈwən ˌweɪ/)


• A man (/mæn/)


• An angel (/ˈeɪnʤəl/)


• An information (/ˌɪnfərˈmeɪʃən/)

When used before an acronym, the rule is still valid, but which article to use depends from how the acronym is pronounced (letter by letter, or as a word).

• An MP3 (/ɛm pi θri/)


• An RPG (/ɑːr pi ʤi/)


• An FBI agent (/ɛf biː aɪ/)


• A GPS device (/ʤi pi ɛs/)


• A NASA employee (/ˈnæsə/)

Note: Some of this information may be extraneous, but take it for what you will!

In general, some acronyms represent nouns, others verbs or adjectives. If it represents the former, I see no problem with prefixing with an (in)definite article (a/an).

scuba is listed as a noun (lower-case) rather than an acronym in most dictionaries these days. It is of course derived from an acronym, but has evolved into a word in its own right (laser would be another example).

FAQ is an acronym, but is very commonly used as a noun - "a list of frequently-asked questions".

FUBAR has various definitions, but it's normally interpreted as an adjective (at least by the original military one).

Hence, I would happily prefix scuba/SCUBA with a/an, but definitely not FUBAR.

All these words begin with hard consonants, and thus should always be prefixed with a. Saying that, some people pronounce FAQ by spelling out its letters, in which case an is appropriate. I've never heard this done with the other two.

Vincent McNabb is correct. If you want evidence based on "credible and/or official sources" that this is the rule followed in formal English writing, here is my suggestion.

I ran Google searches on Google Books only, meaning the bulk of the search will be against professionally edited and published works, not random web sites. I searched only for phrases where the pronunciation of the acronym was relatively clear and consistent: for example, nobody pronounces "SCUBA" as "Ess Cee Yew Bee Ay" and nobody pronounces "FBI" as "Fibbi."

Here are the number of hits in the Google Books database for:

• "A SCUBA": 49,800


• "An SCUBA": 56

• "A FBI": 16,000


• "An FBI": 343,000

• "A NASA": 264,000


• "An NASA": 16,500

• "A RGB": 7,130


• "An RGB": 33,800

• "A UPC": 11,800


• "An UPC": 436

In each case, basing the article on the initial sound, rather than on the initial letter, is more common; in most cases substantially more common.

As a control, I also looked at two acronyms where both the initial sound and the initial letter are consonants.

• "A VPN": 50,100


• "An VPN": 960

• "A OCR": 9,380


• "An OCR": 1,870,000

Because "An VPN" and "A OCR" are incorrect based on any possible rule, we can conclude that the positive results are grammatical, OCR, or search engine errors. This suggests that the minority viewpoint on SCUBA, FBI, NASA, RGB and UPC are also smaller than they appear.

We can conclude that, based on evidence of usage among published documents digitized by Google Books, the preferred rule is to base the article on how the intended pronunciation of the acronym would be spelled phonetically.

It doesn't make any difference at all whether the article is modifying an acronym, an initialism, a proper noun, a French borrowing, or anything else. English article form is determined solely and entirely by pronunciation. And not at all by spelling.

The rule for the pronunciation of articles in English -- definite and indefinite -- is that they have one form before consonants (note, real consonants -- sounds -- not "letters" in a writing system), and a different form before vowels (ditto).

Hence, how you say it is what counts. Nothing else does.

• Before vowels -- Indefinite an /ən/ and Definite the /ði/:
  an hour, an SOS, an A-to-Z selection, an EE degree, an idiot
  the hour, the SOS, the A-to-Z selection, the EE degree, the idiot (all pronounced /ði/)




• Before consonants -- Indefinite a /ə/ and Definite the /ðə/:
  a URL, a snafu, a Charlie Foxtrot, a moron
  the URL, the snafu, the Charlie Foxtrot, the moron (all pronounced /ðə/)

Most native English speakers never notice that there are two different pronunciations for the, but non-native English speakers need to know this immediately.

Like @Vincent McNabb said, it is a question of whether the word is used as an initialism (like HTML) or a acronym. When in doubt, as with FAQ, I would defer to the initialism form ("a FAQ") as it suggests in Wikipedia:

There is also some disagreement as to what to call abbreviations that some speakers pronounce as letters and others pronounce as a word. For example, the terms URL and IRA can be pronounced as individual letters: /ˌjuːˌɑrˈɛl/ and /ˌaɪˌɑrˈeɪ/, respectively; or as a single word: /ˈɜrl/ and /ˈaɪərə/, respectively. Such constructions, however—regardless of how they are pronounced—if formed from initials, may be identified as initialisms without controversy.

Edit:
I originally posted this answer to the question Is there an exceptional use of the article ‘a/an’? which has been merged with this one. The acronyms FTA and FC I refer to below are from that question.

Original answer:
It is exactly as you said: an is used before words beginning with a vowel sound, not necessarily a vowel letter.

The acronyms you mentioned both begin with vowel sounds (/ɛf.tiˈeɪ̯/, /ɛfˈsiː/), so an is used before them. There are also words and acronyms that begin with a vowel letter, but not with a vowel sound: a UAV (/ju.eɪ̯ˈviː/), a union (/ˈjuːnjən/).

It depends on the pronunciation of the following word (not its spelling) whether you use a or an.

Compare with an umbrella uttered as /əmˈbrelə/, versus a university uttered as /juːnɪˈvɜːsɪtɪ/. Though both begin with the same written vowel, they have a different sound at the beginning. Umbrella has a schwa, while university has a 'you'. It is the sounds rather that the written vowel that drive the choice of 'a' or 'an'.

It is similar with your question. "An Eff Tee Ay" or "an Eff See". It is the spoken'Eff', uttered with a vowel sound as /ef/, that drives the use of 'an' even though [f] is listed as a consonant.