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 rule (parolmeans oral; it is related to parliament andparly—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.