As an example, part 91.101 skips every even number until 91.126.
Why are they not in sequence as 101, 102, 103, etc?
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Some of them were renamed/reorganized in 1989 based on proposals/petitions by the AOPA (referenced in FSIMS).
91.102 became 91.143 Flight limitation in the proximity of space flight operations.
91.104 became 91.141 Flight restrictions in the proximity of the Presidential and other parties
91.106/108/110/112/114 did not exist
91.116 became 91.175 Takeoff and landing under IFR
91.118/120/122/126/128 did not exist
The "did not exist" ones are probably proposed legislation that was removed prior to it being introduced into the Code of Federal Regulations, or regulations that were removed earlier (I can't find an older version of the CFR to check).