In legal theory, a default rule is a rule of law that can be overridden by a contract, trust, will, or other legally effective agreement.
Whereas the default rules can be modified by agreement of the parties, mandatory rules will be enforced, even if the parties to a contract attempt to override or modify them.
One of the most important debates in contract theory concerns the proper role or purpose of default rules.
The idea of a default rule in contract law is sometimes connected to the notion of a complete contract.
This is often stated pragmatically as whether a court will imply terms so as to save a contract from uncertainty.