The Parol Evidence Rule

The Parol Evidence Rule forbids allowing oral statements (evidence) to contradict a written contract. The purpose is clear: once an agreement has been reduced to writing, only a subsequent modification in writing should be allowed to modify the contract in the interest of finality and clarity. Without the rule, both written agreements and oral agreements concerning the same contract would have equal weight. Is this rule just? Is the rule necessary to clarify and finalize agreements?