The International Civil Aviation Organization (ICAO) is a UN organization that regulates global aviation standards and practices. The ICAO requires that all pilots and air traffic controllers working on international routes are proficient in English pronunciation, structure, vocabulary, fluency, comprehension and interaction. After reaching the Operational level, pilots and air traffic controllers may operate internationally, but must retake the test every few years until they reach Expert level. The ICAO also recommends that pilots and air traffic controllers use a standard English script. They should avoid using idioms and slang, and should use standard pronunciation and aviation codes for numbers and letters (e.g. Alpha means A and Echo means E).