The discretionary power of a federal court to permit the assertion of a related state law claim, along with a federal claim between the same parties, properly before the court, provided that the federal claim and the state law claim derive from the same set of facts.
Generally, in the civil law, claims based on federal law are heard in federal court, and claims based on state law are heard in state court. The principle of pendent jurisdiction creates an exception to this general rule by allowing a plaintiff who has filed a claim based on federal law in federal court to add a state law claim to the case. This may be done only if the state law claim arose out of the same transaction or occurrence, or nucleus of facts, that gave rise to the federal claim.
For example, assume that a plaintiff has filed suit in federal court alleging that the respondent has violated her civil rights under the civil rights act of 1964 (42 U.S.C.A. § 2000a et seq.). Assume further that the claim arises from an incident in which the plaintiff was denied service at a public restaurant based on her perceived national origin. If the plaintiff was also physically harmed by the respondent in the incident, she may want to file claims for assault and battery. Assault and battery of a private party are state law claims; no federal laws exist under which the plaintiff could bring such claims. Pendent jurisdiction would give the federal court the authority to hear the assault and battery claims because they arose out of the same incident that gave rise to the federal civil rights claims.
Pendent jurisdiction is a rule of judicial convenience and efficiency. If federal courts could not hear state law claims, many plaintiffs would be forced to present two cases in two courts involving essentially the same matter. Such a rule would be unduly expensive for plaintiffs, would increase the number of cases in the court system, and could lead to seemingly inconsistent results from different courts concerning related matters.