In the United States, all commercial planes begin with the letter "N"; a series of unique five-digit numbers follows, and the combination forms a unique registration number. Military aircraft have a slightly different registration format. Registration numbers on those planes follow an "xx-yyyy" format; the "x"s indicate the year in which the planes were placed into service, and the following three or four "y"s show the fleet number.
Private and commercial planes use an alphanumeric system for identification. The Federal Aviation Administration uses "N" as the code for planes registered and operating in the United States, and assigns a unique combination of over 915,000 registration numbers to planes. Older planes, aircraft registered before 1948, have an additional letter in their registration numbers, which identifies their specific category. A "C" indicates a commercial jet, "G" stands for glider, "R" indicates planes used for restricted purposes, "L" indicates a limited plane, "S" stands for a state plane and "X" indicates an experimental aircraft.
Regardless of plane type and age, all aircraft must prominently display a registration number. This number appears near the fuselage or below the tail. Numbers are affixed to a fireproof panel so that they can be retrieved following a crash or fire. Although registration numbers cannot be removed, they can be changed. Planes, like cars, may change ownership, title and nationality, which in turn necessitates a new, unique number.