§ 58 Conventional Custom
Custom which does not fulfil all the requirements hitherto considered by us does not necessarily fail of all legal effect. It cannot, indeed, operate as a source of law by virtue of its own inherent authority. Yet it may nevertheless become legally operative by being incorporated into agreements, through the tacit consent of those who make them. Customs so operative may be distinguished as conventional. It is a rule of English law, as well as of other systems, that where a contract is made in any matter in respect of which an established custom exists, it must be interpreted by reference to that custom, and the parties must be deemed to have intended (in the absence of any expression of contrary intent) to adopt it as one of the terms of their agreement. In contractibus tacite veniunt ea quae sunt moris et consuetudinis.146
For example, if a lease of agricultural land is made in any district in which there are established usages as to the mode of agriculture and as to the relative rights and liabilities of landlord and tenant, the parties must be taken to have agreed to those usages as terms of the bargain, unless they have expressly or implicitly shown an intention to the contrary. In the same way, a mercantile contract must be taken to incorporate any usages of trade which are relevant to its subject-matter. In this manner customs which are not in themselves authoritative as sources of law or rights may become indirectly operative through the added authority of agreement. But the law and rights so produced are in reality conventional and not customary. It is sometimes not easy to determine whether a custom is operative directly and as such, or only indirectly as accessory to a contract, and the distinction has not always been sufficiently adverted to.