Final answer:
The parol evidence rule in common law dictates that written contracts contain all the terms of an agreement and external evidence is not admissible to change or contradict the written terms.
Step-by-step explanation:
The common law rule stating that the contract is the sum of the agreement and that there are no outside agreements to the written document is known as the parol evidence rule. This rule means that when parties have finalized a contract in writing, any previous oral or written statements that do not appear in the contract are generally inadmissible in court if there's a dispute regarding the agreement.
The intention is to ensure that the contract's written terms are honored, and it prevents parties from trying to introduce evidence of alleged side agreements that aren't documented within the contract itself.