A, an

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This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

tl;dr: Use 'an' when a vowel sound follows, 'a' when s consonant sound does.

Sound, not character; in writing too, it should be based on pronunciation. This makes it depend on accent and dialect a little. For example: the h in herb is often silent in American (an herb), but not in British English variants (a herb). Similarly, 'a hotel', unless you pronounce it like 'otel'. In such cases, one form is usually (strongly) preferred over the other.

When writing, there are a few cases that should make you stop and check, including:

An h, when it is silent:

  • an honest man

A u that is pronounced like a y:

  • a used napkin
  • a unique parameter
  • a US ship

An o that gets peonounced like a w:

  • a one-pegged pirate

Note that anything subject to unusual pronunciations could be an exception case. Consider also initialisms/acronyms:

  • an MP
  • a US ship
  • an LBA
  • an SMTP server

Note that when acronyms have varying pronunciations (e.g. URL, LED), both a an an can be correct.