The intention of the parties to a contract is ascertained by applying its terms to the subject matter. Parol evidence is admissible for the purpose of removing any ambiguity which arises in so applying a written contract. The admission of paroi evidence for such purpose does not violate the rule which makes the written instrument the proper and only evidence of the agreement. If the previous negotiations make it manifest in what sense the terms of the contract are used, such negotiations may be resorted to as furnishing the best definition to be applied in ascertaining the intention of the parties. The sense in which the parties understood and used the terms of the contract is thus best ascertained. Stoops v. Smith, 100 Mass. 63. Miller v. Stevens, 100 Mass. 518. Swett v. Shumway, 102 Mass. 365.
In the case at bar, the black lead contracted for was to be paid for by the pound, and the written agreement of the defendants was to take 600 casks in shipments of 100 casks per month. The contract is silent as to the weight of the casks, and therefore the number of pounds which the defendants were obliged to take can only be ascertained by finding what the parties understood by the term “ casks.” The damages which the plaintiff is entitled to recover can only be computed by knowing the weight of the casks named in the contract. It is open to both parties to show by all the circumstances, including what the parties may have said before, or at the time of the contract, or by their admissions afterwards, what was meant by that term. This the *90defendants tried to do in the evidence offered by them and excluded by the judge; and, for this exclusion of evidence, the entry must be Exceptions sustained.