Tail numbers, also known as N-numbers in the United States, are used to identify aircraft. The tail number is usually painted on the rudder or vertical stabilizer of an aircraft, and it may also be placed on other parts of the aircraft's exterior.
In the United States, N-numbers are assigned by the Federal Aviation Administration (FAA). The FAA issues these numbers sequentially, and they are required to be displayed on all civil aircraft that are registered in the country. Most countries have a similar system for numbering aircraft.
The purpose of tail numbers is to help identify aircraft in case of an accident or incident. By having a unique identifier for each plane, authorities can quickly determine which airplane was involved in an incident and track down its owner or operator. Tail numbers can also be used to help keep track of an airplane's maintenance history and airworthiness status.