SET-OFF, in law, is the amount of the debt due to a defendant from a plaintiff, which the defendant is entitled to set off in answer either to the whole or part, as the case may be, of the plaintiff's demand. At common law, if the plaintiff was indebted to the defendant in an ascertained sum in respect of the same transaction concerning which the action was brought, the defendant was entitled to deduct at the trial so much from the plaintiff's demand. But if the debt due from the plaintiff accrued in respect of another transac tion, the defendant had no such power; and lie was either compelled to bring an action against the plaintiff for what was due to him, or, if he wished to avail himself of his cross-demand withont bringing another action, to apply after the action had been commenced against him to a court of equity for the purpose of adjusting the claim, of himself and the plaintiff. To obviate the expense and inconvenience of such a course, it was enacted by 2 Geo. II. c. 22, s. 13, that " where there are mutual debts between the plaintiff and defendant, one debt may be set against the other ;" upon which statute, as explained by S Geo. II. c. 24,s. 4, is founded the whole law on the subject. A claim for damages not ascertained cannot be set 6ff, even although they relate to the subject-matter of the action itself. For instance, in an action
for goods delivered, the defendant cannot set off the leas which he has suffered by their non-delivery at the proper time, Sc. There must also be an entire mutuality between the debt sued for and the set-off. Thus a debt due from the plaintiff together with other parties cannot be set off against a debt due to the plaintiff alone; nor can a debt due to the defendant personally be set off to a demand against him as executor, &c.: and the rule of law is the same for the converse of these cases. It is consistent with this rule that when an action is brought by or against a trustee, a set-off may be made of money due to or from the party for whom he is trustee.
A plea of set-off must describe the debt intended to be set off with the same certainty and particularity as would be nin a decla ration, which indeed such a plea in several respects resembles. If for instance a plea of set-off contains several parts stating distinct debts, these are analogous to distinct counts in a declaration, and ono may be sustained although the others may not.