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.
When we pass from consideration of the words of the contract to the question of what else may be regarded as a term of such contract we are met with a practical difficulty which admits of only a rather arbitrary solution. The parties who enter into a contract do so with full knowledge and in contemplation of a great many rules of law, customs, and facts which they do not carry into their written contract in express terms. They have probably reached their written contract as the result of long negotiations. All these things have undoubtedly made some impression on their minds, and probably have influenced the wording of the contract. To what extent will the law recognize them as terms of the written contract ? The principles of the law which afford a solution of this question are grouped under the rather unfortunate name of the Parol Evidence rule. It may be said to be a general rule that the surrounding facts and circumstances other than the prior and contemporaneous oral negotiations of the parties constitute a part of the contract as long as such contract is not inconsistent therewith.1