Substantive law defines rights and duties, such as crimes and punishments in the criminal law and civil rights and responsibilities in civil law. It is codified in legislated statutes, can be enacted through the initiative process, and in common law systems it may be created or modified through precedent.

Substantive law stands in contrast to procedural law, which is the "machinery" for enforcing those rights and duties. Procedural law comprises the rules by which a court hears and determines what happens in civil or criminal proceedings, as well as the method and means by which substantive law is made and administered.

However, the way to this clear differentiation between substantive law and, serving the substantive law, procedural law has been long, since in the Roman civil procedure the actio included both substantive and procedural elements (see procedural law).

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