The International Civil Aviation Organization uses codes to designate airplane manufacturers, individual airports, airlines and countries. The agency is part of the United Nations, and it develops international standards for civil aviation practices.
The manufacturer's code designates generic aircraft types. For instance, the code ACE stands for Ace Aircraft Manufacturing and Supply in the United States. Manufacturer codes can be any length, and they normally contain some aspect of the company's name. More than 1,850 manufacturer codes exist, as of March 2015.
Airport codes follow a four-letter format to identify individual airports. The first two letters usually designate the country code. The continental United States uses the letter K followed by a three-letter code from the International Air Transport Association. For example, the code KBHM designates Birmingham-Shuttlesworth International Airport in Birmingham, Alabama.
Airline designators have a three-letter code assigned to airlines, aeronautical authorities and airline services. The code AAY stands for Allegiant Air based in Las Vegas. Any agency that operates aircraft gets one of these codes. Each code has a main database entry that lists a base airport, home country and the year during which the airline began operating.
Nationality codes define countries and their dependents. The two-letter country code for the United States is US, while the three-letter code is USA. The three-digit number for America is 840.