Acronyms and initialisms: "a" vs "an" [duplicate]

I was asking this question on Area 51: "How do I tell if an airport scanner is a X-ray scanner?", but I keep wanting to put an 'an' in front of X-ray because it starts with the 'eh' sound.

So is it 'a' or 'an'?

Definitely "an". The word X-ray is never pronounced any way other than "exray", and as has been discussed before, the choice of a or an is based solely on pronunciation, regardless of spelling. Since X-ray is pronounced beginning with a vowel, it must be preceded by an.

'an', because how it sounds is what matters.

An interesting example is an hour, et. al. The fact that h is not considered a vowel from the article point of view says a lot about English pronunciation.

Another interesting example of article form leaking unusual information, from Life, the Universe and Everything:

"I think," said Ford in a tone of voice which Arthur by now recognized as one which presaged something utterly unintelligible, "that there's an SEP over there."

It means that the author meant that SEP achronym is to be pronounced 'es ee pee' and never 'sep'.

In this situation you use "an", because phonetically the sound you are making (the X) starts with an "E": you hear "Ex - ray".

This is true of any letter that, when pronounced, sounds like it starts with a vowel, and often you will need to adapt your use of "a" or "an" even for the same letter.

For example:

A Nasty cold was going around the office

vs

An NHS doctor was the first to diagnose the bug

Phonetically you say "EN AITCH ESS", so you use "An NHS". Likewise, as you say "EX-RAY", you say, "An X-Ray".

The way the word is pronounced is the key factor here, rather than the spelling.

Edit: Looks like JSBangs beat me to it :)