Party Wall, in law, a dividing wall between lands of different proprietors, used in common for the support of structures on both sides. At the common law an owner who has occasion to build on the line of his premises has no right to go beyond the exact line of division between himself and his neighbor, unless he has the neighbor's assent so to do. Nor, though he should erect a wall for his own buildings which is capable of being used by the adjoining proprietor, can he compel such proprietor, when he shall build next to it, to pay any portion of the cost of such wall. But on the other hand, the adjoining proprietor has no right to make any use whatever of such wall without the consent of the owner, and the consequence may be the erection of two walls side by side where one would answer all purposes. This inconvenience is often obviated by an agreement under which a wall for common use is erected, one half of which is on the land of each proprietor, and the expense is borne and the use shared equally; or if only one is to build at the time, the wall may be constructed by him at his own expense, but on the understanding that the other shall pay half the cost when he builds.

Under such an agreement each has an easement in the land of the other while the wall stands, and this accompanies the title in 'sales and descent. But if the wall is destroyed by decay or accident, the easement is gone unless by deed such a contingency is provided for. Eepairs to party walls are to be borne equally, but if one has occasion to strengthen or improve them for more extensive buildings than were first com-templated, he cannot compel the other to divide with him this expense. In some states there are statutes regulating rights in party walls, and one may undoubtedly acquire rights by prescription in a wall built by another which he has long been allowed to use for the support of his own structures.