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A misnomer in that actually two Constitutional clauses are involved. The U.S. Constitution's Article 1 Section 9, C.3 states: 'No Bill of Attainder or ex post facto Law shall be passed,' and Section 10 says: 'No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law. . . .'
The 'words and the intent' of the Ex Post Facto Clause encompass '[e]very law that changes the punishment, and inflicts a greater punishment, than the law annexed to the crime, when committed.' Calder v. Bull, 3 U.S. (1 Dall.) 386, 390 (1798) (opinion of Chase, J.).
An ex post facto law is a law passed after the occurrence of an event or action which retrospectively changes the legal consequences of the event or action.