Contract law: Contract interpretation - Parol Evidence Rule

The parol evidence rule is a rule in the Anglo-American common law that governs what kinds of evidence parties to a contract dispute can introduce when trying to determine the specific terms of a contract. The rule also prevents parties who have reduced their agreement to a final written document from later introducing other evidence, such as the content of oral discussions from earlier in the negotiation process, as evidence of a different intent as to the terms of the contract. The rule provides that "extrinsic evidence is inadmissible to vary a written contract". The term "parol" derives from the Anglo-Norman French p a r o l or p a r  o l e, meaning "word of mouth" or "verbal", and in medieval times referred to oral pleadings in a court case.

The rule's origins lie in English contract law, but has been adopted in other common law jurisdictions; however there are now some differences between application of the rule in different jurisdictions. For instance, in the US, a common misconception is that it is a rule of evidence (like the Federal Rules of Evidence), but that is not the case; whereas in England it is indeed a rule of evidence.

The supporting rationale for this is that since the contracting parties have reduced their agreement to a single and final writing, extrinsic evidence of past agreements or terms should not be considered when interpreting that writing, as the parties had decided to ultimately leave them out of the contract. In other words, one may not use evidence made prior to the written contract to contradict the writing.