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 preceding discussion of the parol evidence rule,1 it appears that at law, and in most cases in equity, the real agreement between the parties, if differing in terms from the written contract, can never be enforced. Extrinsic evidence may overthrow the contract as a whole,2 or it may be used to show some form of subsequent discharge,3 but no method has thus far been considered by which the real agreement which is often back of the written contract can be enforced. Accordingly, a discussion of some of the general principles of Reformation is necessary, since by means of this form of relief, equity can in proper cases and under proper limitations, unhampered by the parol evidence rule, enforce the oral contract which the parties, through mistake in the expression, have not reduced to writing correctly.*