|Call signs consist of a prefix, numeral and a suffix.
CALL SIGN PREFIX:
All U.S. amateur radio call signs contain one or two prefix letters beginning with K, N, W, AA-AL, KA- KZ, NA-NZ or WA-WZ. By agreement, these prefix letters are allocated to the United States by the International Telecommunication Union (ITU.)
CALL SIGN NUMERAL:
This digit is always a single numeral, 0 (zero) through 9 which usually indicates a geographical area within the continental (lower 48 contiguous) United States. For initially issued call signs, the mailing address of the licensee determines the numeral. Area digits (also between 0 and 9) of ham stations outside of the contiguous U.S. are arbitrarily assigned by the FCC.
For example the area digit for American Samoa is the number 8.
The regions and numerals in a U.S. amateur station call sign are: (States are indicated as two letter state postal codes. The regions and call sign digits are the same for Regions 1 through 9.)
For stations located within the continental United States:
1 -- CT, ME, MA, NH, RI and VT
2 -- NJ and NY
3 -- DE, DC, MD and PA
4 -- AL, FL, GA, KY, NC, SC, TN and VA
5 -- AR, LA, MS, NM, OK and TX
6 -- CA
7 -- AZ, ID, MT, NV, OR, UT, WA and WY
8 -- MI, OH and WV
9 -- IL, IN and WI
0 -- CO, IA, KS, MN, MI, NE, MO, ND and SD
For stations outside of the continental 48 United States.
Alaska - Any numeral 1 through 0, zero. (KL9KAA-KL9KHZ is reserved for assignment to U.S. personnel stationed in Korea.)
Caribbean (Atlantic) Insular area. - The numeral 1 indicates Navassa Island; 2 indicates Virgin Islands; 3 or 4 indicates Commonwealth of Puerto Rico except Desecheo Island; and 5 indicates Desecheo Island.
Hawaii and Pacific Insular areas. - The numeral 1 indicates Baker or Howland Island; 2 indicates Guam; 3 indicates Johnston Island; 4 indicates Midway Island; 5 indicates Palmyra or Jarvis Island; 5 followed by suffix letter K indicates Kingman Reef; 6 or 7 indicates Hawaii except Kure Island; 7 followed by the letter K indicates Kure Island; 8 indicates American Samoa; 9 indicates Wake, Wilkes,
or Peale Island; and 0 indicates the Commonwealth of Northern Mariana Islands.
CALL SIGN SUFFIX:
The suffix can be one, two or three alphabet letters. Single-letter suffixes are all letters A through Z. Two-letter combinations are all between AA and ZZ. Three letter combinations are AAA through ZZZ. Some letter combinations are not used such as common Q-signals (example: QST), distress symbols (like SOS) and certain other combinations. The letter X also may not be the first of three suffix
letters in Group D (two-by-three) call sign. By law, those suffixes go to Experimental rather than Amateur radio stations.
When all call signs within a block have been assigned, the next assignment is made from the next consecutive block within a group. When all blocks assigned to a group have been allocated, call signs from the next lower group are assigned. This system is known as the Sequential Amateur Station Call Sign System.