Usage, Usance

"Legal Lexicon":
USAGE, USANCE - Long and uniform practice. In its most extensive meaning this term includes custom and prescription, though it differs from them in a narrower sense, it is applied to the habits, modes, and course of dealing which are observed in trade generally, as to all mercantile transactions, or to some particular branches of trade.
Usage of trade does not require to be immemorial to establish it; if it be known, certain, uniform, reasonable, and not contrary to law, it is sufficient. But evidence of a few instances that such a thing has been done does not establish a usage.
The usages of trade afford ground upon which a proper construction may be given to contracts. By their aid the indeterminate intention of parties and the nature and extent of their contracts arising from mere implications or presumptions, and act of an equivocal character may be ascertained; and the meaning of words and doubtful expressions may become known.
Courts will not readily adopt these usages, because they are not unfrequently founded in mistake.
USANCE - The term usance comes from usage, and signifies the time which by usage or custom is allowed in certain countries, for the payment of a bill of exchange.
The time of one, two or three mouths after the date of the bill, according to the custom of the places between which the exchanges run.
Double or treble is double or treble the usual time, and half usance is half the time. Where it is necessary to divide a month upon a half usance, which is the case when the usance is for one month or three, the division, notwithstanding the difference in the length of the months, contains fifteen days.
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