|Advanced Class operators may only apply for a "Group B, C or D" (2-by-2, 1-by-3 or a 2-by-3) call sign format. A Group B call sign has 2 prefix letters followed by any numeral 0 (zero) through 9 and any two available suffix letters. The prefix must be from the KA to KZ, NA to NZ or the WA to WZ prefix blocks -- but not the AA-AK prefix block which are Group A format call signs. (For example: KA1AA).
The following two letter prefixes are not available to radioamateurs with continental U.S. (the lower 48 contiguous states) mailing addresses: AH, KH, NH, or WH (these are reserved for the Pacific area); AL, KL, NL, or WL (reserved for Alaska) or KP, NP or WP (reserved for the Atlantic area.) There are a few other call signs that may not be selected.