Parol Evidence Rule

The parol evidence rule is a substantive rule of contract law (often misclassified as evidentiary) holding that when parties reduce their agreement to a final written contract, prior or contemporaneous oral statements and prior writings cannot be used to contradict, vary, or add to the terms of that writing. It protects the integrity of written agreements and reflects a presumption that the writing represents the parties' complete bargain.

The rule applies only when the writing is integrated, meaning the parties intended it as a final expression of the terms it contains. Courts distinguish between partial integration (final as to the terms it covers, but supplementable with consistent additional terms) and complete integration (final and exclusive, barring even consistent additional terms). A merger clause (e.g., "This document constitutes the entire agreement of the parties") is strong but not conclusive evidence of complete integration.

Several well-established exceptions allow extrinsic evidence:

• To show the contract is void or voidable for fraud, duress, mistake, illegality, or lack of consideration
• To interpret ambiguous terms
• To establish a condition precedent to the contract's existence
• To prove a collateral agreement supported by separate consideration
• To show subsequent modifications (the rule only bars prior or contemporaneous evidence)

In the United States, the rule is codified for sales of goods in UCC § 2-202, which permits supplementation by course of dealing, usage of trade, and course of performance, and by consistent additional terms unless the writing was intended as complete and exclusive. The Restatement (Second) of Contracts §§ 209–218 sets out the common-law framework.

English law applies a similar principle, sometimes called the parol evidence rule or the "four corners" doctrine, though courts increasingly admit context for interpretation following Investors Compensation Scheme Ltd v West Bromwich Building Society (1998). Civil law systems generally do not apply an equivalent bar, treating all evidence of party intent as admissible subject to ordinary weight assessment.