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How does procedural law differ from substantive law?

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Mosaic portrait of Emperor Justinian I with a jeweled crown and halo, from the 6th-century Basilica of San Vitale in Ravenna, Italy. — Procedural law Justinian I, detail of a 6th-century mosaic at the Church of San Vitale, Ravenna, Italy.

The distinction between procedural law and substantive law lies in their respective functions within the legal system.

Substantive law defines and regulates the legal rights and duties of individuals and collective bodies. It establishes the "what" of the law—for example, defining the elements of a contract or the types of offenses that are punishable under criminal law. These rights, such as those concerning life, liberty, or property, are held to exist for their own sake as part of the normal legal order.123

Procedural law, by contrast, governs the "how." It provides the machinery of the courts and the methods used to enforce the rights established by substantive law. It prescribes the rules for jurisdiction, pleading, evidence, and the execution of judgments.1

In practice, if substantive law describes how two parties enter into a contract, procedural law explains the specific steps a party must take to seek the court's help if that contract is breached. Without this formal, systematic enforcement, the legal system would be inefficient and prone to bias.1