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.
The parol evidence rule has but a limited application to contracts and memoranda which are incomplete on their face. Extrinsic evidence is admissible to show the other terms of such a contract as far as consistent with the terms in writing.1 As far as such a contract is incomplete on its face, it is not within the meaning of the parol evidence rule.2 So where a written contract was made by a widow to take $10,000, and the amount given her by will, in lieu of the distributive share of her husband's estate, and the contract was not complete on its face, it was permissible to show additional terms of the contract, and to show what parties had assented thereto.3 So where a written assignment of a chose in action is incomplete, the oral contract under which it was given may be shown.4 So extrinsic evidence is admissible to show the conditions of an escrow ;5 that the vendee knew of the possession by a squatter of the realty sold ;6 that the amount of notes given included not only the purchase price of the realty conveyed, but also other claims,7 whether a stock option includes dividends or not,8 and whether in a memorandum for the sale of a quarry "with all the improvements thereon" the parties had agreed upon the sale of any of the personal property used in connection therewith.9 If the contract is not required to be in writing or to be proved by writing, and it consists of several writings, no one of which is complete in itself, they may be connected by oral evidence.10 Extrinsic evidence is not admissible to show oral terms inconsistent with those reduced to writing.11 Analagous to the rule that an oral provision consistent with an incomplete written memorandum may be proved and enforced is the rule that if the written contract is ambiguous, the parol evidence rule does not prevent the parties from relying on the real contract, though oral, as long as it does not contradict terms of the written contract which are plain and unequivocal.12 So if the provisions of the written contract admit, a similar result is reached by holding that the written contract will be construed as having the same scope as the oral contract in pursuance of which it is entered into.13
1 Chamberlain v. Lesley, 39 Fla. 452; 22 So. 736; Forsyth Mfg. Co. v. Castlen, 112 Ga. 199; 81 Am. St. Rep. 28; 37 S. E. 485; Louisville, etc., Ry. v. Reynolds, 118 lnd. 170; 20 N. E. 711; Dietrich v. Stebbins, 100 la. 426; 69 N. W. 564; Peneix v. Rodgers (Ky.), 49 S. W. 447; Gould v. Excelsior Co., 91 Me. 214; 64 Am. St. Rep. 221; 39 Atl. 554 ; Courtney v. Mfg. Co., 97 Md. 499; 55 Atl. 614; Stahelin v. Sowle, 87 Mich. 124; 49 N. W. 529; Beyer-stedt v. Mill Co., 49 Minn. 1; 51 N. W. 619; State v. Cunningham, 154 Mo. 161; 55 S. W. 282; Bell v. Wiltson (Neb.), 98 N. W. 1049; Jamestown Business Association v. Allen, 172 N. Y. 291; 92 Am. St. Rep. 740; 64 N. E. 952; Virginia-Carolina Chemical Co. v. Moore, 61 S. C 166; 39 S. E. 346; Waterbury v. Russell, 8 Baxt. (Tenn.) 159; Howell v. Denton (Tex. Civ. App.), 68 S. W. 1002; Steed v. Harvey, 18 Utah 367; 72 Am. St. Rep. 789; 54 Pac. 1011; Knowles v. Rogers, 27 Wash. 211; 67 Pac. 572; Seeger v. Boiler Co., - Wis. -; 97 N. W. 485; Naumann v. Ullman, 102 Wis. 92; 78 N. W. 159.
2 Sloan v. Courtenay, 54 S. C. 314; 32 S. E. 431.
3 Baldwin v. Hill, 97 la. 586; 66 N. W. 889.
4 Randall v. Turner, 17 O. S. 262, and see Sec. 1199.
5 Fred v. Fred (N. J. Eq.), 50 Atl. 776.
6 Leonard v. Woodruff, 23 Utah 494; 65 Pac. 199.
7Brader v. Brader, 110 Wis. 423; 85 N. W. 681.