However, if the case is brought as a diversity action (i.e., the basis for federal jurisdiction is that each defendant comes from a state different from each plaintiff), there generally is no supplemental jurisdiction if such claims would destroy complete diversity.
See Exxon Mobil Corp. v. Allapattah Services, Inc. Courts are also free to decline to exercise supplemental jurisdiction in specified or exceptional circumstances (§ 1367(c)).
Pendent jurisdiction is the authority of a United States federal court to hear a closely related state law claim against a party already facing a federal claim, described by the Supreme Court as "jurisdiction over nonfederal claims between parties litigating other matters properly before the court.
"[3] Such jurisdiction is granted to encourage both "economy in litigation",[4] and fairness by eliminating the need for a separate federal and state trial hearing essentially the same facts yet potentially reaching opposite conclusions.
Pendent jurisdiction refers to the court's authority to adjudicate claims it could not otherwise hear.
The leading case on pendent jurisdiction is United Mine Workers of America v. Gibbs, 383 U.S. 715 (1966).
Gibbs has been read to require that (1) there must be a federal claim (whether from the Constitution, federal statute, or treaty) and (2) the non-federal claim arises "from a common nucleus of operative fact" such that a plaintiff "would ordinarily be expected to try them in one judicial proceeding."
Moore v. New York Cotton Exchange and Owen Equipment & Erection Co. v. Kroger are seminal cases relating to ancillary jurisdiction.