The best interpreter of things is usage
Lord coke says that ancient charters, whether before the time of memory or not, ought to be construed as the law was taken when the charter was made, and according to ancient allowance and, that when any claimed before the justices in eyre any franchises by ancient charter, though it had express words for the franchises claimed ; or, if the words were general, and a continual possession pleaded of the franchises claimed ; or, if the claim was by old and obscure words, and the party in pleading expounded them to the court, averring continual possession according to that exposition ; the entry ever was, “Inquiratur super possessionem et usum” agreeable to that old rule, “Optimus interpres rerum usus.”
The custom of the country with respect to the right of the tenant or lessee to take away growing crops at the expiration of the term, and as to the mode of cultivation of the lands in lease, must be considered as impliedly annexed to the terms of a lease, unless expressly excluded ; and this is in accordance with the maxim under consideration. By custom, in some districts the outgoing tenant is bound to leave upon the premises a certain quantity of clover and grass seeds, or fallow, or turnips, or hay and straw, or manure, or to consume all the hay and straw upon the premises, and many other such like conditions ; all which will, in the construction of any contract of tenancy, be considered as forming part of it, unless expressly excluded ; and parol evidence of the custom and usage is always admissible to ascertain the rights and liabilities of the parties to the contract. But parol evidence of custom and usage will not be admitted to nullify the 'express provisions of such contract. The same rule applies to mercantile contracts and usages.
This maxim may not inaptly be called a creature of circumstance, and the reason of it, a state of things acquiesced in rather than agreed to, the law of times of ignorance, and indifference ; and though old customs still remain, and habit and practice, for convenience of people and encouragement of commercial enterprise, assume with us the name of custom ; yet, written law is, in modern times, gradually assuming the ascendancy over, if not the total abrogation of, custom. Custom, however, whether particular or general, is law, and usage is evidence of custom. Common or general custom is the common law of the country, and particular custom the particular law of the place, person, or thing to which it applies.
There are, however, some limits to a custom. For example, it must be obligatory, reasonable, and certain. It must not be against the good of the public, nor the many, and in favour of a few, or one person. It must have existed, without interruption, from time immemorial. And, lastly, it cannot prevail against a public statute, or express contract inter partes.
The following maxims also are applicable to this : “Consuetudo ex certa, causa rationabili usitata privat communem legem” A custom proceeding from certain reasonable use supersedes the common law ; but, “Consuetudo, licit sit magnae auctoritatis, nunquam tamen praejudicat manifestae veritati”. A custom, though allowed upon great authority, should never be permitted to prejudice manifest truth. The maxim, “Modus et conventio vincunt legem” may also be considered in connection with this.