A or An? Its a question that is now confused with the increasing amount of communication done purely in text. In spoken English, it is quite simple: 'A' before a noun, 'An' before a vowel. Spoken is the key word here - the spoken language doesn't care if its 'ewe' or 'you', 'won' or 'one'. In written English, the vowels are defined as 'a', 'e', 'i', 'o' and 'u'. Thus, two words that sound the same may mistakenly have two different ways to write them: 'an ewe' and 'a you', or 'a won' and 'an one'. Try saying these word pairs out loud and it is immediately apparent that 'an ewe' and 'an one' sound wrong. There are other examples that exist, 'http' is typically pronounced 'eitch-tee-tee-pee' and thus starts with a vowel sound - 'a http' does not sound right at all. Likewise 'SGML' is pronounced 'ess-jee-em-ell' and sounds wrong with 'a' in front of it.

The key to deciding which indefinite article is correct is to say the word and listen to the first sound. The a/an distinction was created for the spoken language. Even though the rules do not fit well with many people's ideas of how to write text, this is how it is meant to be.

Webster gives several very good examples of this in a and an.

On a similar note, I've noticed a difference in use of a/an before abbreviations. I don't think there are any "official" rules on this, because it's never a problem orally.

Let's look at an example.

In my work, we talk about System-on-a-Chip, commonly abbreviated to SOC. Consider a textbook saying one of the following:

a) Blocks of a SOC are connected ...
b) Blocks of an SOC are connected ...

As far as I can see, both are perfectly legal. The difference is how the author would pronounce the abbreviation. SOC would be pronounced respectively:

a) sock
b) ess-oh-see

Further to the above, before words that begin with an unstressed syllable that begins with a 'h', such as 'historian' and 'hotel', you may (but don't have to) use 'an': 'an historian', 'an hotel' (or 'a historian', 'a hotel').

And there are also some words which will differ in different places: some people (such as myself) pronounce aich with a haich, so would say 'a http'; others don't. Americans, i have been led to believe, say 'herb' without the 'h', so an American might write "color of an herb," whereas a non-American might prefer 'colour of a herb'.

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