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. Contract law, for example, can be divided into two kinds of rules: default rules and mandatory rules. 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.