Parol Evidence Rule

The PER bars prior or contemporaneous terms that contradict a final written integration.

Representative cases

• Houston Exploration Co. v. Wellington Underwriting Agencies, Ltd. 352 S.W.3d 462 · Texas Supreme Court
• Union Oil Co. v. Newton 245 N.W.2d 11 · Michigan Supreme Court
• Astor v. Boulos Co., Inc. 451 A.2d 903 · Supreme Judicial Court of Maine

Related Contract Content rules

• Parol Evidence ExceptionsParol evidence is admissible to explain ambiguities, show defenses, or prove collateral agreements.
• Interpretation: Course of Dealing/Trade UsageAmbiguous terms may be interpreted using course of performance, dealing, and trade usage.
• Merger Clause EffectA merger clause indicates full integration, strengthening application of the parol evidence rule.

Part of Contracts on Core Legal Rules — foundational MBE-tested principles with primary-source citations.