Originally Posted By: ILBob
2.36" = 60 mm though. that seems like an arbitrary number from some country using the metric system.

From what I understand, the TSA is trying to be consistent with other, international guidelines. The 6 cm rule appears to be in use by the Eurpean Union and I've read that it is a part of the security standards adopted by the UN's International Civil Aviation Organization.
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Victory awaits him who has everything in order — luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck. Roald Amundsen