Each station is assigned a call sign which is used to identify the station during transmissions. Amateur station call signs in the US take the format of one or two letters (the prefix), then a numeral (the call district), and finally between one and three letters (the suffix). The number of letters used in the call sign is determined by the operator's license class and the availability of letter combinations. The format of the callsign is often abbreviated as X-by-X where a number in place of the X indicates the quantity of letters, separated by a single digit of the call district. Currently there are 13 geographically based regions. There were 9 original call districts within the 48 contiguous states, also known as radio inspection districts. The 10th district (with numeral 0) was split from the 9th district. Three additional regions cover Alaska, the Caribbean (including Puerto Rico), and the Pacific (including Hawaii). In the last few decades the FCC has discarded the requirement that a station be located in the corresponding numerical district. Whereas at one time the callsign W1xxx would have been solid identification that the station was in New England (district 1), that is no longer the case, and W1xxx may be located anywhere in the USA. Even particularly distinctive calls such as KH6xxx which used to be exclusively in Hawaii, may be assigned to license holders on the US mainland. However, those licensees with KH6, KL7, KP4, etc., call signs must have been living (or had a mailing address) in Hawaii, Alaska or Puerto Rico when they received those call signs. A newly licensed amateur will always receive a call sign from the district in which they live. For instance, a newly licensed Technician from New England would receive a call sign of the form KC1xxx. The amateur may thereafter apply for a specific or specialized call sign under the Vanity Licensing program. Approximately 88% of all amateur radio operators have call signs that reflect the district in which those operators live. An amateur operator with an Amateur Extra Class license can hold a call from any of the four call sign groups, either by keeping an existing call sign (indefinitely, since there is no requirement to change call sign upon license renewal), or by choosing a Group B, C or D call sign under the Vanity Licensing Program. Likewise, Advanced Class licensees can hold Group