It was done recently. Dozens of people weighed in. Quick and simple answer: it depends on whether the initial phoneme of the spoken form is vowel or consonant in nature. SLA, spoken, is “Ess Ell Ay” – and hence takes “an.” BLT, on the other hand, is “Bee Ell Tee” and takes “a.” Notice that four vowels and numerous consonants begin with a vowel sound, but U, Y, and numerous other consonants with a consonant sound. Hence the usage is “a USCG cutter” because the oral form is “You Ess See Gee.”
On acronyms that are said as words, like radar and NATO, the same rule applies – the initial phoneme of the pronounced word is what governs. “A NATO base typically has a radar station, guarded by a SEALs detachment or an RAF detachment.”
I thought there was a recent thread too, but I can’t find it. The conclusion is the use “a” or “an” depending on how the acronym/initialism is pronounced, not how it is spelled.
So “an SLA”.
"Use an in place of a when it precedes a vowel sound, not just a vowel. That means it’s “an honor” (the h is silent), but “a UFO” (because it’s pronounced yoo eff oh). This confuses people most often with acronyms and other abbreviations: some people think it’s wrong to use “an” in front of an abbreviation (like “MRI”) because “an” can only go before vowels. Poppycock: the sound is what matters. It’s “an MRI,” assuming you pronounce it “em ar eye.”
It’s the same as words, really. You would say “an honor,” not “a honor,” as well as “a universe,” not “an universe.” The first letter of the following word never determines whether a or an should be used; it’s the sound.
What is the grammatical rule which governs the following statement recorded in a policeman’s notebook:
‘When I read the defendant his rights he threatened to insert an (unlit) firework up my left nostril.’
I thought, and I may be wrong, that if any word or phrase in brackets is removed from a sentence then what remains must make grammatical sense. In this case it doesn’t. However ‘a (unlit) firework’ doesn’t seem to work either.
Not quite: The policeman, not required to enforce prescriptivist “Laws of Usage,” chose “an” on the basis of what he had actually written, which included “(unlit)” as the next-following word. In edited renderings of quotations, you’ll find all manner of parenthetic material that when removed renders the sentence much less sensible. “He [said] that he [would have] finished by next Monday.” This is clearly a case in which tenses have been changed to fit the surrounding narrative. But omitting the words in brackets leaves the initial clause without a verb and turns the future perfect construction into a bizarre past-for-future tense.