[l] Litt. §. 212.
[m] 1 Inst. 62.
[n] Co. Copyh. §. 33.
5. Customs ought to be certain. A custom, that lands shall descend to the most worthy of the owner's blood, is void; for how shall this worth be determined? But a custom to descend to the next male of the blood, exclusive of females, is certain, and therefore good[o]. A custom, to pay two pence an acre in lieu of tythes, is good; but to pay sometimes two pence and sometimes three pence, as the occupier of the land pleases, is bad for it's uncertainty. Yet a custom, to pay a year's improved value for a fine on a copyhold estate, is good: though the value is a thing uncertain. For the value may at any time be ascertained; and the maxim of law is, id certum est, quod certum reddi potest.
[o] 1 Roll. Abr. 565.
6. Customs, though established by consent, must be (when established) compulsory; and not left to the option of every man, whether he will use them or no. Therefore a custom, that all the inhabitants shall be rated toward the maintenance of a bridge, will be good; but a custom, that every man is to contribute thereto at his own pleasure, is idle and absurd, and, indeed, no custom at all.
7. Lastly, customs must be consistent with each other: one custom cannot be set up in opposition to another. For if both are really customs, then both are of equal antiquity, and both established by mutual consent: which to say of contradictory customs is absurd. Therefore, if one man prescribes that by custom he has a right to have windows looking into another's garden; the other cannot claim a right by custom to stop up or obstruct those windows: for these two contradictory customs cannot both be good, nor both stand together. He ought rather to deny the existence of the former custom[p].
[p] 9 Rep. 58.
Next, as to the allowance of special customs. Customs, in derogation of the common law, must be construed strictly. Thus, by the custom of gavelkind, an infant of fifteen years may by one species of conveyance (called a deed of feoffment) convey away his lands in fee simple, or for ever. Yet this custom does not impower him to use any other conveyance, or even to lease them for seven years: for the custom must be strictly pursued[q]. And, moreover, all special customs must submit to the king's prerogative. Therefore, if the king purchases lands of the nature of gavelkind, where all the sons inherit equally; yet, upon the king's demise, his eldest son shall succeed to those lands alone[r]. And thus much for the second part of the leges non scriptae, or those particular customs which affect particular persons or districts only.
[q] Co. Cop. §. 33.