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Airlines use the three-letter codes internationally in their own network, Sita, for messages such as passenger loads and departure times. World ATC and weather agencies use a separate teleprinter network, the Aeronautical Fixed Telecommunications Network (AFTN), which uses a four-letter "location indicator." Going from large area to actual airport, the first letter relates to the part of the world and the second letter the country. The third letter is a group of airports within that country. Most countries who use this particular convention use a letter to denote the FIR in which the airport is located. So F is Frankfurt FIR in Germany, M is Munich; P is Paris FIR, M is Marseilles. Other ways to use the third letter include identifying a group of airports with a common factor. For example, A was used in Germany for all Canadian and American air force bases. The last letter positively identifies a specific airport.
Thus Aberdeen, Scotland, has the International Civil Aviation Organization (ICAO) location indicator of EGPD -- E for Northern Europe, G for United Kingdom, P for Scottish region, and D for Dyce field. Want to figure out LFPG? It's L for southern Europe, F for France, P for Paris FIR, and G for Charles de Gaulle airport. Easy! One more example is EDMM. E for northern Europe, D for Deutchland (Germany), M for Munchen (Munich) FIR, and M again for the Munich airport.
So if London Heathrow has two codes -- and it does, LHR and EGLL -- how come I've heard Chicago O'Hare only called ORD? The answer is unique to the United States. Here the ICAO code is formed simply by adding a "K" to a Sita code. This explains why international flight plans refer to KORD, KMIA, KJFK, etc.