the special and particular customs of
manors, of which every one has more or less, and which bind all the
copyhold-tenants that hold of the said manors.--Such likewise is the
custom of holding divers inferior courts, with power of trying causes,
in cities and trading towns; the right of holding which, when no royal
grant can be shewn, depends entirely upon immemorial and established
usage.--Such, lastly, are many particular customs within the city of
London, with regard to trade, apprentices, widows, orphans, and a
variety of other matters; which are all contrary to the general law of
the land, and are good only by special custom, though those of London
are also confirmed by act of parliament[a].

[Footnote a: 8 Rep. 126. Cro. Car. 347.]

TO this head may most properly be referred a particular system of
customs used only among one set of the king's subjects, called the
custom of merchants or _lex mercatoria_; which, however different from
the common law, is allowed for the benefit of trade, to be of the
utmost validity in all commercial transactions; the maxim of law
being, that "_cuilibet in sua arte credendum est_."

THE rules relating to particular customs regard either the proof of
their existence; their legality when proved; or their usual method of
allowance. And first we will consider the rules of proof.

AS to gavelkind, and borough-english, the law takes particular notice
of them[b], and there is no occasion to prove that such customs
actually exist, but only that the lands in question are subject
thereto. All other private customs must be particularly pleaded[c],
and as well the existence of such customs must be shewn, as that the
thing in dispute is within the custom alleged. The trial in both cases
(both to shew the existence of the custom, as, "that in the manor of
Dale lands shall descend only to the heirs male, and never to the
heirs female;" and also to shew that the lands in question are within
that manor) is by a jury of twelve men, and not by the judges, except
the same particular custom has been before tried, determined, and
recorded in the same court[d].

[Footnote b: Co. Litt. 175 _b._]

[Footnote c: Litt. Sec. 265.]

[Footnote d: Dr and St. 1. 10.]

THE customs of London differ from all others in point of trial: for,
if the existence of the custom be brought in question, it shall not be
tried by a jury, but by certificate from the lord mayor and aldermen
by the mouth of their recorder[e