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 by its terms applies only to prior and contemporaneous negotiations. Contracts which are in writing merely because the parties thereto chose to reduce them to writing offer no technical difficulties to subsequent oral modifications. Accordingly the parol evidence rule does not prevent the parties to a written contract, not under seal, and not required by law to be in writing or to be proved by writing, from making subsequent oral modifications of its terms.1 Thus a subsequent oral settlement making an account stated,2 a subsequent extension of time,3 a subsequent agreement that a policy, the premium for which by its terms was payable in advance, should take effect at once, the insurer holding the policy until the premium was paid;4 to deliver a note to an agent of the adversary party,5 or providing that a note already endorsed should be received as security and not as payment6 may all be used as modifications of prior written contracts. A provision in a written contract that no one can change its provisions,7 or that they can be changed only by writing,8 are each ineffectual to prevent subsequent oral modifications. So a subsequent agreement by a vendor, on valuable consideration, to give different warranties from those in the original written contract of sale can be enforced.9 The subsequent modification can be most readily shown after it has been performed in full.10 By statute in some jurisdictions, subsequent oral modifications of written contracts can be enforced only when partly performed. If purely executory they are unenforceable.11 The oral modification is not partly performed unless something has been done which the party performing was not bound to do under the original contract.12 Subsequent conversations as to the meaning of a prior contract, not amounting to a new contract and not giving rise to an estoppel, are not intended to change the legal effect of such contract and hence do not operate as such change.13
5colder v. Dobell, L. R. 6 C. P. 486; Bateman v. Phillips, 15 East. 272; Obiter in Higgins v. Senior, 8 M. & W. 834; York County Bank v. Stein, 24 Md. 447.
1Wood v. Ft. Wayne. 119 U. S. 312; The Sappho, 94 Fed. 545; 36 C. C. A. 395; reversing 89 Fed. 366; Pecos Valley Bank v. Evans-Snider-Buel Co., 107 Fed. 654; 46 C. C. A. 534; Andrews v. Tucker. 127 Ala. 602; 29 So. 34; Hartford, etc.. Co. v. Attalla, 119 Ala. 59; 54 So. 845; Katz v. Bedford, 77 Cal. 319; 1 L. R. A. 826; 19 Pac. 523; Hurl-burt v. Dusenbery, 26 Colo. 240; 57 Pac. 860; Gunby v. Drew. - Fla. -; 34 So. 305; Chicago, etc., Co. v. Moran, 187 Ill. 316; 58 N. E. 335; affirming 85 Ill. App. 543; Palmer v. Bennett. 96 Ill. App. 281: Toledo, etc., Ry. v. Levy, 127 Ind. 168;
26 X. E. 773; Bartlett v. Stanch-field. 148 Mass. 394; 2 L. R. A. 625: 19 X. E. 549; Mouat v. Bam-let, 123 Mich. 345; 82 X. W. 74; Moore v. Locomotive Works, 14 Mich. 266; Conrad v. Fisher. 37 Mo. App. 352; 8 L. R. A. 147; Harris v. Murphy, 119 X. C. 34; 56 Am. St. Rep. 656; 25 S. E. 708; Wadge v. Kittleson, - X. D. -; 97 X. W. 856; Cline v. Shell. 43 Or. 372; 73 Pac. 12; Cunningham v. Church, 159 Pa. St. 620; 2S Atl. 490; Chicago, etc., Co. v. Barry (Tenn. Ch. App.), 52 S. W. 451; Carstens v. Earles, 26 Wash. 676; 67 Pac. 404. See Ch. LXII.
2 Krueger v. Dodge, 15 S. D. 159; 87 X. W. 965.
3 Bannon v. Aultman, 80 Wis. 307: 27 Am. St. Rep. 37; 49 X. W. 967.
4 Prudential Ins. Co. v. Sullivan, 27 Tnd. App. 30; 59 N. E. 873.
5 Stokes v. Polley, 164 N. Y. 266; 58 N. E. 133.
6 Willow River Lumber Co. v. Furniture Co., 102 Wis. 636; 78 N. W. 762.
7 Peterson v. Reaping Machine Co.. 97 la. 148; 59 Am. St. Rep. 399; 66 N. W. 96.
8 Chicago, etc., R. R. v. Moran, 187 Ill. 316; 58 N. E. 335; Illinois Central Ry. v. Manion, - Ky. -; 67 S. W. 40.
9 McCormick Harvesting Mach. Co. v. Hiatt, - Neb. Rep. Unofficial -; 95 N. W. 627.
10 Town v. Jepson, - Mich. -; 95 N. W. 742.
11 Armington v. Stelle, 27 Mont. 13; 69 Pac. 115.
12 Mackenzie v. Hodgkin, 126 Cal. 591; 77 Am. St. Rep. 209; 59 Pac. 36.
13 Dixon v. Williamson, 173 Mass. 50; 52 N. E. 1067.