the exception that proves the rule
US /ðɪ ɪkˈsɛpʃən ðæt pruːvz ðə ruːl/
UK /ðɪ ɪkˈsɛpʃən ðæt pruːvz ðə ruːl/
1.
a case that does not follow a rule but, by its very existence, shows that the rule is generally true
:
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Most people here are quiet, but John is very outgoing; he's the exception that proves the rule.
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My boss usually doesn't allow working from home, but he let me today because of my appointment. It's the exception that proves the rule.