What legal principle allows a counterclaim in a federal district court?

A counterclaim in federal district court is allowed under the principle of supplemental jurisdiction. This legal principle permits a court to hear and decide claims that may not independently qualify for federal jurisdiction if those claims are closely related to other claims that do fall within the court’s jurisdiction.

In the context of a counterclaim, this means that if a defendant raises a counterclaim against the original plaintiff, and that counterclaim is related to the original dispute (such as arising out of the same transaction or occurrence), the federal district court can exercise supplemental jurisdiction over that counterclaim even if it wouldn’t independently meet the requirements for federal jurisdiction, such as diversity or arising under federal law.

While the other options reference various aspects of jurisdiction, they do not specifically address the ability to bring a counterclaim in conjunction with a related claim. Jurisdiction of the original plaintiff pertains more to the initial authority of the court to hear the plaintiff's claim. Exclusive federal jurisdiction refers to cases solely under the purview of federal law, while concurrent jurisdiction indicates that both federal and state courts can hear certain types of cases. However, none of these concepts specifically facilitate the function of allowing a counterclaim to be added in a federal lawsuit, emphasizing the unique role of supplemental jurisdiction in this context.