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13.2 The Parol Evidence Rule

Learning Objectives

  1. Understand the purpose and operation of the parol evidence rule, including when it applies and when it does not.
  2. Know how the Uniform Commercial Code (UCC) deals with evidence to show a contract’s meaning.

The Purpose of the Rule

Unlike Minerva sprung forth whole from the brow of Zeus in Greek mythology, contracts do not appear at a stroke memorialized on paper. Almost invariably, negotiations of some sort precede the concluding of a deal. People write letters, talk by telephone, meet face-to-face, send e-mails, and exchange thoughts and views about what they want and how they will reciprocate. They may even lie and cajole in duplicitous ways, making promises they know they cannot or will not keep in order not to kill the contract talks. In the course of these discussions, they may reach tentative agreements, some of which will ultimately be reflected in the final contract, some of which will be discarded along the way, and some of which perhaps will not be included in the final agreement but will nevertheless not be contradicted by it. Whether any weight should be given to these prior agreements is a problem that frequently arises.

Parol Evidence at Common-Law

The Rule

The rule at common law is this: a written contract intended to be the parties’ complete understanding discharges all prior or contemporaneous promises, statements, or agreements that add to, vary, or conflict with it.

The parol evidence ruleUnder this rule, where there is a written contract, extrinsic (parol) evidence cannot usually change the express terms laid down in that document. (parol means oral; it is related to parliament and parly—talking) is a substantive rule of law that operates to bar the introduction of evidence intended to show that the parties had agreed to something different from what they finally arrived at and wrote down. It applies to prior written as well as oral discussions that don’t make it into the final written agreement. Though its many apparent exceptions make the rule seem difficult to apply, its purposes are simple: to give freedom to the parties to negotiate without fear of being held to the consequences of asserting preliminary positions, and to give finality to the contract.

The rule applies to all written contracts, whether or not the Statute of Frauds requires them to be in writing. The Statute of Frauds gets to whether there was a contract at all; the parol evidence rule says, granted there was a written contract, does it express the parties’ understanding? But the rule is concerned only with events that transpired before the contract in dispute was signed. It has no bearing on agreements reached subsequently that may alter the terms of an existing contract.

The Exemptions and Exceptions

Despite its apparent stringency, the parol evidence rule does not negate all prior agreements or statements, nor preclude their use as evidence. A number of situations fall outside the scope of the rule and hence are not technically exceptions to it, so they are better phrased as exemptions (something not within the scope of a rule).

Not an Integrated Contract

If the parties never intended the written contract to be their full understanding—if they intended it to be partly oral—then the rule does not apply. If the document is fully integrated, no extrinsic evidence will be permitted to modify the terms of the agreement, even if the modification is in addition to the existing terms, rather than a contradiction of them. If the contract is partially integrated, prior consistent additional terms may be shown. It is the duty of the party who wants to exclude the parol evidence to show the contract was intended to be integrated. That is not always an easy task. To prevent a party later from introducing extrinsic evidence to show that there were prior agreements, the contract itself can recite that there were none. Here, for example, is the final clause in the National Basketball Association Uniform Player Contract: “This agreement contains the entire agreement between the parties and there are no oral or written inducements, promises or agreements except as contained herein.” Such a clause is known as a merger clauseA contract term stating that the written agreement contains—merges—the parties’ full understanding and intent..

Void or Voidable Contracts

Parol evidence is admissible to show the existence of grounds that would cause the contract to be void. Such grounds include illegality, fraud, duress, mistake, and lack of consideration. And parol evidence is allowed to show evidence of lack of contractual capacity. Evidence of infancy, incompetency, and so on would not change the terms of the contract at all but would show it was voidable or void.

Contracts Subject to a Condition Precedent

When the parties orally agree that a written contract is contingent on the occurrence of an event or some other condition (a condition precedentA term in a contract that something has to happen before the obligation to perform the contract ripens.), the contract is not integrated and the oral agreement may be introduced. The classic case is that of an inventor who sells in a written contract an interest in his invention. Orally, the inventor and the buyer agree that the contract is to take effect only if the buyer’s engineer approves the invention. (The contract was signed in advance of approval so that the parties would not need to meet again.) The engineer did not approve it, and in a suit for performance, the court permitted the evidence of the oral agreement because it showed “that in fact there never was any agreement at all.”Pym v. Campbell, 119 Eng. Rep. 903 (Q.B. 1856). Note that the oral condition does not contradict a term of the written contract; it negates it. The parol evidence rule will not permit evidence of an oral agreement that is inconsistent with a written term, for as to that term the contract is integrated.

Untrue Recital or Errors

The parol evidence rule does not prevent a showing that a fact stated in a contract is untrue. The rule deals with prior agreements; it cannot serve to choke off inquiry into the facts. Thus the parol evidence rule will not bar a showing that one of the parties is a minor, even if the contract recites that each party is over eighteen. Nor will it prevent a showing that a figure in the contract had a typographical error—for example, a recital that the rate charged will be the plumber’s “usual rate of $3 per hour” when both parties understood that the usual rate was in fact $30 per hour. A court would allow reformationThe correction of a contract containing errors. (correction) of such errors.


To enforce a contract, its terms must be understood, so parol evidence would be allowed, but a claim of ambiguity cannot be used to alter, vary, or change the contract’s meaning.

Postcontract Modification

Ordinarily, an additional consistent oral term may be shown only if the contract was partially integrated. The parol evidence rule bars evidence of such a term if the contract was fully integrated. However, when there is additional consideration for the term orally agreed, it lies outside the scope of the integrated contractA contract that encompasses the parties’ full understanding. and may be introduced. In effect, the law treats each separate consideration as creating a new contract; the integrated written document does not undercut the separate oral agreement, as long as they are consistent. Buyer purchases Seller’s business on a contract; as part of the agreement, Seller agrees to stay on for three weeks to help Buyer “learn the ropes.” Buyer realizes she is not yet prepared to go on her own. She and Seller then agree that Seller will stay on as a salaried employee for five more weeks. Buyer cannot use the parol evidence rule to preclude evidence of the new agreement: it is a postcontract modification supported by new consideration. Similarly, parties could choose to rescind a previously made contract, and the parol evidence rule would not bar evidence of that.

The UCC Approach

Under Section 2-202 of the UCC, a course of dealing, a usage of trade, or a course of performance can be introduced as evidence to explain or supplement any written contract for the sale of goods. A course of dealingA pattern of behavior between parties showing how they intend their relationship to work. is defined as “a sequence of previous conduct between the parties to a particular transaction which is fairly to be regarded as establishing a common basis of understanding for interpreting their expressions and other conduct.” A usage of tradeCustomary way of doing business that may be used to inform the parties’ contractual intentions. is “any practice or method of dealing having such regularity of observance in a place, vocation or trade as to justify an expectation that it will be observed with respect to the transaction in question.” A course of performanceSystematic and uniform conduct in which parties engage after they enter into a contract. is the conduct of a party in response to a contract that calls for repeated action (e.g., a purchase agreement for a factory’s monthly output, or an undertaking to wash a neighbor’s car weekly).

Key Takeaway

The parol evidence rule is intended to preserve “the four corners” of the contract: it generally prohibits the introduction of contemporaneous oral or written elements of negotiation that did not get included in the written contract, subject to a number of exemptions.

The UCC allows evidence of course of dealing, course of performance, or usage of trade to give meaning to the contract.


  1. What is the purpose of the parol evidence rule?
  2. How does it operate to crystallize the intention of the contracting parties?
  3. To what kinds of contract issues does the rule not apply?
  4. What “help” does the UCC give to fleshing out the parties’ contractual understanding?