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.
As far as the parol evidence rule itself is concerned, it makes no difference what the subject-matter of the contract is, with what formalities the contract has in fact been executed, or with what formalities it is required by law to be executed in order to be operative. In itself it applies, primarily, to contracts which the parties have reduced to writing voluntarily, and which would have been operative although such contracts had not been reduced to writing and although no written evidence of such contract had ever been in existence. Other rules than the parol evidence rule may, however, apply to contracts which involve certain classes of subject-matter and which are required by law to be in writing or to be proved by writing or to be executed with other formalities. While the parol evidence rule applies to contracts of these classes as well as to ordinary contracts, as far as it operates to exclude extrinsic evidence, parol evidence is frequently excluded by reason of the application of these other rules of law in cases in which the parol evidence rule itself would permit the admission of such evidence. If the contract is one of a type which by law must be in writing, or which by law must be proved by writing, the rules of law which require such contract to be proved by writing or to be in writing may prevent the use of oral evidence for the purpose of identifying the parties or the subject-matter, or for the purpose of explaining ambiguities, or for the purpose of supplementing a written memorandum or contract which is incomplete upon its face.
1Hurless v. Wiley, 91 Kan. 347, L. R. A. 1915C, 177, 137 Pac. 981.
2 See Sec. 2165.
3 See Sec. 2046.
4 "We prefer, however, to put the decision on this ground: If originally the buyer would not have been permitted to show that at the time of his purchase, and as an inducement thereto, the seller orally agreed to find a new buyer at an advanced price within the year, the reason is that the law protects the seller from having his documentary evidence overthrown by mere word of mouth. When he reduced the promise to writing, the requirement of the law was met. The reason for his protection no longer existed. Oral evidence was necessary, not to prove the promise, but to show for what it was given. A consideration was implied from the contract being in writing, and its character was properly shown by parol." Hurless v. Wiley, 91 Kan. 347, L. R. A. 1915C, 177, 137 Pac. 981. 5 See Sec. 589.