An aircraft tail number, or registration mark, acts exactly like a license plate for a car, providing a unique identity for every civil aircraft. The first one or two characters are a country prefix assigned by the ICAO; for example, "N" is the United States, "G" is the United Kingdom, "D" is Germany, and "F" is France. The characters following the hyphen or prefix are unique to that specific airframe. In the U.S., these "N-numbers" can be a mix of up to five numbers and letters. Beyond legal identification, tail numbers allow enthusiasts to track a plane's history, age, and owner. For private jets, owners often pay for "vanity plates"—like a Gulfstream with a tail number ending in "GA" for Gulfstream Aerospace. In the cockpit, the tail number is also used as the aircraft's radio callsign unless it is a commercial flight using a specific flight number (e.g., "United 204"), making it a critical part of global air traffic safety.