The common law of every country is made up in great measure of customs which have come to be universally recognized and adopted, of which the courts take judicial notice, and by which they adjudicate the rights and liabilities of individuals. But besides these customs which constitute a part of the general law of the state, special customs also prevail, some of which are confined to particular localities, while others relate to particular trades, professions, or occupations. Thus, there are in England the custom of gavelkind, prevailing in Kent, under which all the sons inherit, instead of the eldest son alone; the custom of borough English in some boroughs, by which the youngest son is favored in the inheritance; and the custom of London, which among other things permits a married woman to engage in mercantile pursuits. These also are judicially noticed by the courts. The custom of merchants is often alluded to as if in a similar way it constituted an exception to the common law, but this is a mistake; it is only one branch of that law which has grown up with commerce and expanded with the changes in the modes of transacting business and the necessities of trade.
But there are also other customs which the common law permits to be established, but of which the courts can have no knowledge until informed by evidence, and which consequently must be proved in any case in which they are relied upon, and be found as facts by the jury. The proof consists in showing immemorial and uniform usage in accordance with the custom relied upon; and this furnishes the definition of custom, as being a usage which by common consent and uniform practice has become the law of the place, or of the subject matter to which it relates. Blackstone gives the several requisites of a good custom as follows: 1. It must have been used so long that the memory of man runneth not to the contrary. If this were strictly correct, it would preclude the establishment of any new customs; but in fact these are constantly being established and supported, and now when an immemorial usage is spoken of, all that is meant is a usage which has prevailed for a sufficient time to raise a reasonable presumption that parties in their dealings have adopted it as a rule of action. 2. It must have been continuous. 3. It must have been peaceable and acquiesced in, and not subject to contention and dispute. 4. It must be reasonable. 5. It must be certain. 6. It must be compulsory, and not left to the option of every man whether he will use it or no. 7. Customs must be consistent with each other; one custom cannot be set up in opposition to another, which would be contradictory and absurd.
To these it may be added that custom cannot set aside or vary the statute law, or be opposed to the constitution of the land, or repugnant to the general principles of the common law. Neither can it be allowed to overcome or vary express contracts. It may explain contracts by establishing the sense in which the words employed in them are understood in the business, and which may be different from the ordinary sense; but it is only enforced in any case upon the presumption that the parties have adopted it in their dealings, which can never arise when they have agreed to the contrary. The reasonableness of a custom is a question of law for the court; and "it will not be enforced or have the sanction of law, unless it be reasonable and convenient, and adapted not only to increase facilities in trade, but to promote just dealings." (9 Met-calf 's Reports, 363.) And any custom opposed to the general law of the state on the subject to which it refers must always be held unreasonable. - The local customs of France were more numerous and diverse during the middle ages than those of England, owing to the greater independence of the French nobles, which left each province more at liberty to originate usages for itself.
Not less than 60 are enumerated as having been reduced to a certainty during the 16th century, besides more than 200 others inconsiderable for extent or peculiarity. These gradually assimilated, and in large degree became absorbed in the body of general law. The coutume de Paris, as the common law of the French colonists in America, prevailed more or less in the states formed from the Louisiana purchase, after French dominion ceased, until expressly abrogated by statute.