A counterclaim is a claim that a defendant may have against a plaintiff. The compulsory counterclaim arises from the same transaction or occurrence that forms the basis of the plaintiff’s suit. However, if the claim is the subject of another pending action and if such action has commenced, then the defendant cannot state the claim. A subsequent action on a matter that might have been included as a counterclaim in the prior action is abated by the prior action[i].
For example, a car accident between A and B leads to a personal injury lawsuit. But the defendant asserts a compulsory counterclaim that the plaintiff actually owes him/her damages for injuries. A compulsory counterclaim generally must be a part of the initial answer to the plaintiff’s action and cannot be made later in the suit or in a separate lawsuit. In other words, if the defendant fails to sue on the claim now, s/he cannot sue in the future on that same claim.
Whereas, permissive counterclaim arises from an event unrelated to the suit of plaintiff. For example, William Jake breaks his leg while visiting the home of Camille Dorson. Jake sues Dorson alleging that Dorson negligently left her child’s roller skate on her front porch which resulted in the accident. In a permissive counterclaim, Dorson asserts that Jake owes her money. The court will rule separately on the respective claims of plaintiff and defendant. This case will involve respective allegations of negligence and bad debt. Permissive counterclaims can be made even at the later stage of the suit or in a different suit.
[i] Friedrichsen v. Cobb, 84 Mont. 238 (Mont. 1929)