a or the see grammar and syntax.
Write words in their full form on first appearance:
Trades Union Congress (not TUC), Troubled Asset Relief Programme (not TARP)
unless an abbreviation or acronym is so familiar that it is used more often in full:
AIDS BBC CIA EU FBI HIV IMF NATO NGO OECD UNESCO
or unless the full form would provide little illumination – AWACS, DNA. If in doubt about its familiarity, explain what the organisation is or does. After the first mention, try not to repeat the abbreviation too often; so write the agency rather than the IAEA, the party rather than the KMT, to avoid spattering the page with capital letters. There is no need to give the initials of an organisation if it ...