An N-number begins with a run of one or more numeric digits, may end with one or two alphabetic letters, may only consist of one to five characters in total, and must start with a digit other than zero. In addition, N-numbers may not contain the letters I or O, due to their similarities with the numerals 1 and 0. Each alphabetic letter in the suffix can have one of 24 discrete values, while each numeric digit can be one of 10, except the first, which can take on only one of nine values. This yields a total of 915,399 possible registration numbers in the namespace, though certain combinations are reserved either for government use or for other special purposes. The following are the combinations that could be used: N1 to N9: Federal Aviation Administration (FAA) internal use only N10 to N99: Federal Aviation Administration (FAA) internal use only N100 to N999 N1000 to N9999 N10000 to N99999 N1A to N9Z N10A to N99Z N100A to N999Z N1000A to N9999Z N1AA to N9ZZ N10AA to N99ZZ N100AA to N999ZZ An older aircraft (registered before 31 December 1948) may have a second letter in its identifier, identifying the category of aircraft. This additional letter is not actually part of the aircraft identification (e.g. NC12345 is the same registration as N12345). Aircraft category letters have not been included on any registration numbers issued since 1 January 1949, but they still appear on antique aircraft for authenticity purposes. The categories were: C = airline, commercial and private G = glider L = limited R = restricted (such as cropdusters and racing aircraft) S = state X = experimental For example, N-X-211, the Ryan NYP aircraft flown by Charles Lindbergh as the Spirit of St. Louis was registered in the experimental category. There is a unique overlap in the United States with aircraft having a single number followed by two letters and radio call signs issued by the Federal Communications Commission to Amateur Radio operators holding the Amateur Extra class license. For example, N4YZ is, on the one hand, a Cessna 206 registered to a private individual in Melba, Idaho, while, on the other hand, is also issued to an Amateur Radio operator in North Carolina. Since an aircraft registration number is also used as its call sign, this means that two unrelated radio stations can have the same call sign. Decolonisation and independence