The Parol Evidence Rule Explained

The parol evidence rule states that when parties have reduced their agreement to a final written document (an “integrated” agreement), evidence of prior or contemporaneous oral or written agreements that contradict or supplement the writing is generally inadmissible. In plain English: if you signed a written contract, you usually can't introduce earlier conversations or drafts to change what the contract says.

The rule's application depends on the type of integration:

Totally integrated: The writing is the complete and final expression of the parties' agreement. No parol evidence may supplement or contradict it.

Partially integrated: The writing is final but not complete — the parties intended to include only some terms. Parol evidence may supplement (add consistent additional terms) but not contradict.

Not integrated: The writing was never intended as a final agreement. The parol evidence rule doesn't apply.

Parol evidence IS admissible to show:

1. Formation defenses: Fraud, duress, mistake, illegality, lack of consideration — these go to whether a valid contract exists at all
2. Ambiguity: To interpret ambiguous terms in the writing
3. Conditions precedent: That the written contract was conditioned on some event
4. Subsequent modifications: Agreements made after the writing (the rule only bars prior/contemporaneous evidence)
5. Collateral agreements: Separate agreements on different subject matter that wouldn't naturally be included in the writing

Under UCC § 2-202, the parol evidence rule is more flexible for sale of goods:

- Course of dealing, usage of trade, and course of performance are always admissible to supplement the writing
- Additional consistent terms are admissible unless the court finds the writing was intended as a complete agreement

This is more permissive than the common law approach and reflects the commercial reality that written contracts rarely capture every term of a business deal.

When you see parol evidence on an exam:

1. Is the writing integrated? (Merger clause = strong evidence of total integration)
2. If totally integrated: is the parol evidence contradicting or supplementing?
3. Does any exception apply?
4. If UCC applies, note the more liberal standard

The most common exam pattern: parties sign a contract with a merger clause, then one party claims there was an oral side agreement. Walk through the analysis systematically.