The Constitution
ex post facto law
A law that criminalizes conduct after it happened, or increases the punishment retroactively. Forbidden to both Congress and the states.
In full
The ban appears twice, in Article I Sections 9 and 10, and applies to criminal law rather than to civil legislation. It bars punishing conduct that was lawful when done, raising a penalty after the fact, or changing the rules of evidence to make conviction easier. Retroactive civil laws are analyzed under due process instead.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.