LEGAL PRINCIPLE: CONTRACT LAW – Written Agreement – Parol Evidence Rule

The parol evidence rule excludes extrinsic (external) evidence regarding written contract terms. When parties reduce agreements to writing: the writing is primary evidence, oral or extrinsic evidence cannot: add terms, vary terms, subtract terms, or contradict terms. This rule serves: certainty (written terms are definitive), protecting against false claims about oral variations, and respecting parties’ decision to formalize agreements in writing. “Extrinsic evidence” includes: oral testimony about different terms, prior negotiations, contemporaneous oral agreements, or subsequent oral modifications. However, limited exceptions permit extrinsic evidence to: prove fraud, mistake, or duress; establish meaning of ambiguous terms; show trade custom or usage; prove conditions precedent to contract effectiveness; or demonstrate terms implied by law. The rule doesn’t prevent: evidence about contract formation, proving the contract was executed, or showing subsequent written amendments. This principle reinforces written agreement sanctity—once parties commit terms to writing, those written terms govern. Oral evidence about contractual terms is generally inadmissible, protecting written agreement integrity.