alt Hacker NewsThe expression is referring to an implicit or unstated rule. Defining it is hard but people know when it has been broken. Hooters is an exception, the rule is, don't be like Hooters.
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timknauf • 11/07/2024
Thank you! I never understood the expression, but this explanation was immediately clarifying for me.
I see from Wiktionary that it was originally a legal concept, expressed in medieval Latin.
https://en.wiktionary.org/wiki/exception_that_proves_the_rul...
Just as you say, the point is that a rule is implied by a specific exception, as in the example "free entry on Sundays", which implies the unstated rule "pay for entry on other days".
The exception weakens the rule, it's true, but may also reveal the rule.