A few countries do not fully comply with these rules. Australian broadcast stations officially have—but do not use—the VL prefix, and Canada uses Chile's CB for its own Canadian Broadcasting Corporation stations.
With regard to the second and/or third letters in the prefixes in the list below, if the country in question is allocated all callsigns with A to Z in that position, then that country can also use call signs with the digits 0 to 9 in that position. For example, the United States is assigned KA–KZ, and therefore can also use prefixes like KW0 or K1.
Many large countries in turn have internal rules on how and where specific subsets of their callsigns can be used (such as Mexico's XE for AM and XH for FM radio and Television broadcasting), which are not covered here.
Unallocated: The following call sign prefixes are available for future allocation by the ITU. (x represents any letter; n represents any digit from 2–9.)
(* Indicates a prefix that has recently been returned to the ITU.)
Unavailable: Under present ITU guidelines the following call sign prefixes shall not be allocated
They are sometimes used unofficially - such as amateur radio operators operating in a disputed territory or in a nation state that has no official prefix (e.g. S0 in Western Sahara or 1A at Knights of Malta headquarters in Rome). (x represents any letter; n represents any digit from 2–9.)
(S19.33 Between radiocommunication conferences, the Secretary-General is authorized to deal with questions relating to changes in the allocation of series of call signs, on a provisional basis, and subject to confirmation by the following conference.)