<P> An ex post facto law (corrupted from Latin: ex postfacto, lit .' out of the aftermath') is a law that retroactively changes the legal consequences (or status) of actions that were committed, or relationships that existed, before the enactment of the law . In criminal law, it may criminalize actions that were legal when committed; it may aggravate a crime by bringing it into a more severe category than it was in when it was committed; it may change the punishment prescribed for a crime, as by adding new penalties or extending sentences; or it may alter the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed . Conversely, a form of ex post facto law commonly called an amnesty law may decriminalize certain acts . A pardon has a similar effect, in a specific case instead of a class of cases . Other legal changes may alleviate possible punishments (for example by replacing the death sentence with lifelong imprisonment) retroactively . Such legal changes are also known by the Latin term in mitius . </P> <P> A law may have an ex post facto effect without being technically ex post facto . For example, when a previous law is repealed or otherwise nullified, it is no longer applicable to situations to which it had been, even if such situations arose before the law was voided . The principle of prohibiting the continued application of such laws is called nullum crimen, nulla poena sine praevia lege poenali, especially in European Continental systems . This is related to the principle of legality . </P>

Where are ex post facto laws discussed and what does it say