An aircraft registration, alternatively called a tail number, is a code unique to a single aircraft, required by international convention to be marked on the exterior of every civil aircraft. The registration indicates the aircraft's country of registration, and functions much like an automobile license plate or a ship registration. This code must also appear in its Certificate of Registration, issued by the relevant National Aviation Authority (NAA). An aircraft can only have one registration, in one jurisdiction, though it is changeable over the life of the aircraft.