parol evidence

The parol evidence rule is the legal application of a rule of substantive law in contract cases that prevents a party to a written contract from contradicting (or sometimes adding to) the terms of the contract by seeking the admission of evidence "extrinsic" (outside) to the contract. For example, Carl agrees in writing to sell Betty a car for $1,000. Betty argues that Carl told her that she would only need to pay Carl $800. The parol evidence rule would generally prevent Betty from testifying to this conversation because the testimony ($800) would directly contradict the written contract's terms ($1,000).

In order for the rule to be effective, the contract in question must first be a final integrated writing; it must, in the judgment of the court, be the final agreement between the parties (as opposed to a mere draft, for example).