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The first number after the hyphen is an aircraft designation. The rule, although often violated, is that normal aircraft are to be designated in a strict numerical series according to their basic mission. The easiest examples are found in the Fighter class of US airplanes: F-14, then the F-15, F-16 and so on. But, there are exceptions. For example, the X-35, which was a research plane, was later redesignated the F-35 when it became fighter capable, even though the next number in the Fighter sequence was F-24.