The parol evidence rule provides that evidence extraneous to a written contract cannot be received to add to, vary or contradict its terms. Although it can be so simply stated, this rule has been the source of a great deal of confusion in the law of contract. It was enforced rigidly when it first became established as part of the common law but has since been gradually relaxed. As the English courts became faced with new situations where too strict an adherence to the rule would have caused injustice, they created numerous apparent exceptions to it. The decisions of the Canadian and other Commonwealth courts have followed a similar pattern. The rule has often been stated and applied in wide exclusionary terms yet, in a whole host of cases, evidence has been admitted to vary or add to, and sometimes even to directly contradict, the terms of a writing.
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David W. McLauchlan, “The Inconsistent Collateral Contract” (1976-1977) 3:1 DLJ 136.