Rule 14.Third-party practice

Rule 14(a) lets a defending party -- as a third-party plaintiff -- serve a summons and complaint on someone not already a party who is or may be liable to the third-party plaintiff for all or part of the original plaintiff's claim, at any time after the action begins. No leave of court is needed if the third-party complaint is filed within 10 days after the original answer is served; after that, the third-party plaintiff needs leave of court, on motion with notice to all parties. The person served this way -- the third-party defendant -- must raise defenses under Rule 12 and any counterclaims or cross-claims under Rule 13, just like any other defendant.

The rule then opens up a web of claims among everyone now in the case. The third-party defendant may assert against the plaintiff any defense the third-party plaintiff has against the plaintiff's claim, and may also bring its own claim against the plaintiff arising from the same transaction or occurrence as the plaintiff's claim. The plaintiff, in turn, may assert a claim against the third-party defendant arising from that same transaction or occurrence, and if it does, the third-party defendant must respond with defenses under Rule 12 and counterclaims or cross-claims under Rule 13, exactly as before. Any party may move to strike the third-party claim or ask for its severance or a separate trial. A third-party defendant can even bring in a further third party under this same rule if that further party is or may be liable to it.

Rule 14(b) extends the same mechanism to a plaintiff: once a counterclaim is asserted against the plaintiff, the plaintiff may bring in a third party under the same circumstances that would let a defendant do so.