Parol Evidence Rule

The parol evidence rule is a substantive rule of contract law that limits the use of extrinsic evidence — oral or written statements made before or at the time of contracting — to contradict, modify, or supplement the terms of a fully integrated written agreement. The rule protects the integrity of the written document and ensures that parties who have reduced their agreement to writing are not subjected to claims about inconsistent prior promises.

The threshold question is whether the writing is integrated — that is, whether the parties intended it to be the final expression of their agreement. A fully integrated writing cannot be supplemented or contradicted by parol evidence. A partially integrated writing cannot be contradicted but may be supplemented by consistent additional terms. If the writing is not integrated at all, parol evidence is freely admissible.

Courts use different approaches to determine integration. The four corners test looks only at the document itself to determine whether it appears complete. The Corbin approach looks at all available evidence, including the disputed parol evidence, to determine whether the parties intended the writing to be a complete integration. The UCC takes a liberal approach, allowing parol evidence to supplement a writing with course of dealing, usage of trade, and course of performance.

Several important exceptions permit parol evidence despite integration: to prove fraud, duress, or mistake; to explain ambiguous terms; to establish a condition precedent to the contract's effectiveness; to show a subsequent modification; and to interpret the meaning of terms using trade usage or course of dealing.

The parol evidence rule intersects with the statute of frauds, merger clauses, and the interpretation of contract terms. A merger clause (also called an integration clause) is a contractual provision stating that the writing is the complete and exclusive statement of the parties' agreement. While merger clauses are strong evidence of full integration, they are not always conclusive.

On exams, parol evidence questions require students to analyze whether the writing is integrated, which version of the rule the jurisdiction follows, and whether any exceptions apply.