This section is from the book "The Law Of Contracts", by Theophilus Parsons. Also available from Amazon: The law of contracts.
A custom which may be regarded as appropriate to the contract and comprehended by it, has often very great influence in the construction of its language. (e) The general reason
(c) Crocker v. The Franklin H. & F. Man. Co. 3 Sumner, 530; Ellis v. Thompson, 3 M. & W. 445; Greaves v. Ashlin, 3 Camp. 426; Sawyer v. Hammatt, 15 Me. 40; Howe v. Huntington, id. 350; Atkinson v. Brown, 20 Me. 67. And see Atwood v. Emery, cited ante, p. *496, note (o).
(d) Attwood v. Clark, 2 Greenl. 249; Kingsley v. Wallace, 14 Me. 57; Murry v. Smith, 1 Hawks, 41. For certain exceptions to this rule, see Howe v. Huntington, 15 Me. 350. See also Hill v. Hobart, 16 Me. 164.
(e) That evidence may be given of a custom or usage of trade to aid in the construction of a contract, either by fixing the meaning of words where doubtful, or by giving them a meaning wholly distinct from their ordinary and popular sense, is a well established doctrine. Thus, where it was represented to underwriters, on a policy of insurance, that the ship insured was to sail "in the month of October," evidence was admitted to show that the expression "in the month of October," was well understood amongst men used to commercial affairs to signify some time between the 25th of that month and the 1st or 2d of the succeeding month. Chaurand v. Angentein, Peake, N. P. 43. So, also, custom or usage may be admitted to show, that a "whaling voyage " includes the taking of sea-elephants, on the beaches of islands and coasts, as well as whales. Child v. Son Mutual Ins. Co. 3 Sandf.26. So also as to the meaning of "cotton in bales." Taylor v. Briggs, 2 C. & P. 525, and Outwater v. Nelson, 20 Barb. 29, as to the phrase "on freight." Evidence may also be admitted, that the word "days" in a bill of lading means working days, and not running days. Cochran v. Retberg, 3 Esp. 121. Evidence may also be given of the mercantile meaning of the terms "good" and "fine," as applied to barley. Hutchison v. Bowker, 5 M. & W. 535; Whit-more v. Coats, 14 Mo. 9. So also as to the meaning of the word "privilege," in an agreement with the master of a ship. Birch v. Depeyster, 4 Camp. 385. In Evans v. Pratt, 3 Man. & G. 759, evidence was admitted to show that "across a country," in a memorandum respecting a race, means that the riders are to go over all obstructions, and are not at liberty to use a gate. See Sleight v. Harts-home, 2 Johns. 531, as to the meaning of of * this is obvious enough. If parties enter into a con tract, by virtue whereof something is to be done by one or both, and this * thing is often done in their neighbor hood, or by persons of like occupation with themselves, and is always done in a certain way, it must be supposed that they intended it should be done in that way. The reason for this supposition is nearly the same as that for supposing that the common language which they use is to be taken in its common meaning. And the rule that the meaning and intent of the parties govern, wherever this is possible, comes in and operates. Hence an established custom may add to a contract stipulations not contained in it; on the ground that the parties may be sup posed to have had these stipulations in their minds as a part of their agreement, when they put upon paper or expressed in words the other part of it. (f) 1 So custom may control and vary
"sea-letter." Astor v. Union In&. Co. 7 Cowen, 202, as to the meaning of "furs." See also Haynes v. Holliday, 7 Bing. 587; Bead v. Granberry, 8 Ired. 109; Barton v. McKelway, 2 N. J. 174; Robertson v. Jackson, 2 C. B. 412; Moore v. Campbell, 10 Excb. 322, 26 Eng. L. & Eq. 522; Vail v. Rice, 1 Seld. 155. So in the case of a contract to sell "mess pork of Scott & Co.," evidence was admitted to show that this language in the market meant pork manufactured by Scott & Co. Powell v. Horton, 2 Bing. N. C. 668. Where a contract was worded thus: "Sold 18 pockets Kent hops, at 100c.," it was permitted to be shown that by the usage of the hop trade, a contract so worded was understood to mean 100s. per cwt. and not per pocket. Spicer v. Cooper, 1 Q.B. 424. See also Bowman v. Horsey, 2 Moody & R. 85. So evidence has been admitted to show that "rice" is not considered as corn within the memorandum of a policy of insurance. Scott v. Bourdillion, 6B.&P. 213. See also Clayton v. Gregson, 5 A. & E. 802, as to the meaning of the word "level" among miners. Also Cuthbert v. Cumming, 11 Ezch. 405, 30 Eng. L. & Eq. 604, as to the phrase "full and complete cargo." And see Grant v. Maddox, 15 M. & W. 787; Brown v. Byrne, 3 Ellis & B. 708,26 Eng. L. & Eq. 247. So as to the meaning of "in regular turns of loading," Liedemann v. Schultz, 14 C. B. 38, 24 Eng. L. & Eq. 806. Owing to the loose and inaccurate manner in which policies of insurance are drawn, a class of cases has sprung up, almost peculiar to this instrument, in which evidence is admitted of usages between the underwriters and the assured, affixing to certain words and clauses a known and definite meaning. Thus, in Brough v. Whitmore, 4 T. R. 206, on evidence of the practice of merchants and underwriters, it was held, that provisions, sent out in a ship for the use of the crew, were protected: by a policy on the ship and furniture. Lord Kenyon, in giving judgment, said: "I remember it was said many years ago, that if Lombard street had not given a construction to policies of insurance, a declaration on a policy would have been had on general demurrer; but that the uniform practice of merchants and underwriters had rendered them intelligible." In Coit v. Commercial Ins. Co. 7 Johns. 385, evidence was received of a usage among underwriters and merchants restricting the term "roots " in the memorandum of a policy to such articles as were in their nature perishable, and excluding sarsaparilla. See also Allegre v. Maryland Ins. Co. 2 Gill & J. 136; 8. c. 6 Harris &, J. 408; Macy v. Whaling Ins. Co. 9 Met. 354; Eyre v. Marine Ins. Co. 5 Watts & S. 116; 1 Duer on Ins. 185; Humphrey v. Dale, 7 Ellis & B. 265; Cuthbert v. Gumming, 11 Exch. 405,30 Eng. L. & Eq. 604.
 
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