An "N-number" is the unique alphanumeric string that serves as the national registration for an aircraft registered in the United States. Just as a car has a license plate, every civil aircraft must display its registration number clearly on the fuselage or tail. The "N" prefix is the International Civil Aviation Organization (ICAO) country code for the United States, a designation that dates back to the early 20th century. Following the "N," there can be a series of up to five characters, which can be numbers, letters, or a combination of both (e.g., N123AB). These numbers are assigned by the Federal Aviation Administration (FAA). Private owners often pay a small fee to "reserve" a custom N-number, similar to a vanity plate. The N-number allows air traffic control, law enforcement, and the public to identify the aircraft's owner, its airworthiness status, and its flight history through the FAA's public database. If a plane is sold to a foreign buyer, the N-number is removed and replaced with the prefix of the new country (e.g., "G" for the United Kingdom or "VH" for Australia). It is a vital part of global aviation security and tracking.