exception that proves the rule

Derived from the medieval Latin legal principle "exceptio probat regulam in casibus non exceptis" ("the exception tests the rule in the cases not excepted").

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exception that proves the rule (plural exceptions that prove the rule or exceptions that prove rules)

1. The rare occurrence of a counterexample to a rule, used to demonstrate that the rule exists.
   • 1688, [anonymous], An Answer to a Paper Importing a Petition of the Archbishop of Canterbury and Six other Bishops to His Majesty‎, London: Henry Hills, page 14:

     A bishop ... is not bound to obey any Mandate but the King's; which Exception proves the Rule, and that he is inexcusably oblig'd to obey the King's

   • 1775, "A Gentleman of Lincoln's Inn", A Digest of Adjudged Cases in the Court of King's Bench‎, London: G. Kearsly and G. Robinson, page 85:

     Some statutes give justices of the peace a power of proceeding on default, but the exception proves the rule in the thing not excepted; it seems therefore that the defendant should have been apprehended by a warrant

• exception proves the rule
• the exception proves the rule

• honor in the breach
• the proof of the pudding is in the eating

• Gary Martin (1997–) “The exception that proves the rule”, in The Phrase Finder, retrieved 26 February 2017.
• Michael Quinion (September 14, 2002) “Exception that proves the rule”, in World Wide Words.
• The Straight Dope

• exceptions that prove the rule