The parol evidence rule keeps earlier or contemporaneous agreements out of the case when the parties have put their deal in a final writing. It is not a rule about oral evidence only, and it is not a rule of evidence at all. It is a substantive rule about which terms are part of the contract. If the writing is the final expression of the deal, an earlier promise that contradicts it is simply not a term, and the jury never hears about it.
Rule
- Ask whether the writing is integrated. A partially integrated writing is final as to what it says. A completely integrated writing is final as to everything.
- Partial integration. Consistent additional terms may be proved, but contradictory terms may not.
- Complete integration. Neither contradictory nor additional terms come in.
- Order matters. The rule only reaches agreements made before or at the same time as the writing. A later modification is never barred by it.
- Exceptions. Evidence is admissible to show fraud, duress, mistake, illegality, a condition precedent to the contract taking effect, or the meaning of an ambiguous term.
Leading cases
- Mitchill v. Lath: a promise to remove an icehouse was excluded, because a term of that kind would ordinarily have been included in the written land contract if the parties had agreed to it.
- Masterson v. Sine: a more liberal approach, admitting evidence of a collateral agreement that might naturally have been made separately by parties in that situation.
- Pacific Gas and Electric Co. v. G. W. Thomas Drayage and Rigging Co.: extrinsic evidence is admissible to show that contract language is reasonably susceptible to the meaning a party urges, even where the words look plain.
Where students go wrong
The first error is applying the rule to later agreements. Anything the parties agreed after signing is a modification, and it stands or falls on its own consideration and formality, not on this rule.
The second is skipping the integration question. Whether the writing is partially or completely integrated decides everything else, and courts split on how to answer it. A merger clause is strong evidence of complete integration but is not always conclusive.
The third is forgetting that interpretation is a separate step. The rule keeps out extra terms; it does not stop a court from working out what the terms that survived actually mean.
FAQ
Does the parol evidence rule apply to oral evidence only?
No. It covers any prior agreement, written or oral, and any oral agreement made at the same time as the writing.
What is a merger clause?
A clause saying the writing is the entire agreement. It is powerful evidence of complete integration, but courts will still look past it where there is fraud or where the clause was boilerplate nobody negotiated.
Can I use parol evidence to prove fraud?
Yes. The rule assumes a genuine agreement. Evidence that the assent was procured by fraud, duress or mistake attacks the writing itself and comes in.
Related terms
Working through this in your own casebook takes longer than reading it here. Syllume starts from your syllabus and turns each assigned case into a brief you can study from.