The block beginning with letter Q is under international telecommunications jurisdiction, but is used internally by FAA Technical Operations to identify National Airspace equipment not covered by any other identifying code system. The block beginning with Z identifies United States Air Route Traffic Control Centers. In practice, the assigned identifiers are not always consistent with the current "encoding" rules adopted by the FAA, nor are all the assigned identifiers unique between the United States and Canada. The coding system has evolved over time, and to ensure safety and reduce ambiguity, many "legacy" codes have remained intact, even though they violate the currently ordered rules. For this reason, the FAA regularly publishes detailed listings of all codes it administers General Assignment Patterns In general, three-letter identifiers are assigned as radio call signs to aeronautical navigation aids; to airports with a staffed air traffic control facility or navigational aid within airport boundary; to airports that receive scheduled route air carrier or military airlift service, and to airports designated by the United States Customs Service as airports of entry. Some of these identifiers are assigned to certain aviation weather reporting stations. Most one-number, two-letter identifiers have been assigned to aviation weather reporting and observation stations and special-use locations. Some of these identifiers may be assigned to public-use landing facilities within the United States and its jurisdictions, which do not meet the requirements for identifiers in the three-letter series. In this identifier series, the number is always in the first position of the three-character combination. Most one-letter, two-number identifiers are assigned to public-use landing facilities within the United States and its jurisdictions, which do not meet the requirements for identifiers in the three-letter series. Some of these identifiers are also assigned to aviation weather reporting stations. One-letter, two-number identifiers are keyed by the alphabetical letter. The letter may appear in the first, middle or last position in the combination of three characters. When the letter signifies an air traffic control center's area, the assignment will not change if the center's boundaries are realigned. Identifiers in this series which could conflict with the Victor, Jet or colored airway numbers are not assigned. Two-letter, two-number identifiers are assigned to private-use landing facilities in the United States and its jurisdictions which do not meet the requirements for three-character assignments. They are keyed by the two-letter Post Office or supplemental abbreviation of the state with which they are