exception that proves the rule

Meaning

Noun

• The rare occurrence of a counterexample to a rule, used to demonstrate that the rule exists.

Origin

• Derived from the medieval Latin legal principle "exceptio probat regulam in casibus non exceptis" ("the exception tests the rule in the cases not excepted").
• {{examples|width=40%|examples=* "Entry is free on Sundays."
  - This implies that entry is not free on the other six days of the week.