This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
From the statement of the parol evidence rule, it evidently can apply only under a combination of certain facts. The rule applies: (1) where there is a complete written contract; (2) in an action between the parties to the contract or their representatives; (3) in an action in which the meaning of the contract is involved directly; (4) where the validity of the contract itself is not in issue; and (5) where an attempt is made to show prior or contemporaneous oral terms of such contract. If any one of these facts is lacking, the parol evidence rule has no application. Accordingly, it is necessary to consider a group of cases where the rule may seem applicable at first glance, but which are on analysis seen to be completely without the very terms of the rule itself.