Although it is sometimes said that there are two separate court systems, the reality is more complex. There are, in fact, fifty-two court systems: those of the fifty states, the local court system in the District of Columbia, and the federal court system. At the same time, these are not entirely separate; they all have several points of contact.

State and local courts must honor both federal law and the laws of the other states. First, state courts must honor federal law where state laws are in conflict with federal laws (under the supremacy clause of the Constitution; see Chapter 4 “Constitutional Law and US Commerce”). Second, claims arising under federal statutes can often be tried in the state courts, where the Constitution or Congress has not explicitly required that only federal courts can hear that kind of claim. Third, under the full faith and credit clause, each state court is obligated to respect the final judgments of courts in other states. Thus a contract dispute resolved by an Arkansas court cannot be relitigated in North Dakota when the plaintiff wants to collect on the Arkansas judgment in North Dakota. Fourth, state courts often must consider the laws of other states in deciding cases involving issues where two states have an interest, such as when drivers from two different states collide in a third state. Under these circumstances, state judges will consult their own state’s case decisions involving conflicts of laws and sometimes decide that they must apply another state’s laws to decide the case (see Table 3.1 “Sample Conflict-of-Law Principles”).

As state courts are concerned with federal law, so federal courts are often concerned with state law and with what happens in state courts. Federal courts will consider state-law-based claims when a case involves claims using both state and federal law. Claims based on federal laws will permit the federal court to take jurisdiction over the whole case, including any state issues raised. In those cases, the federal court is said to exercise “pendent jurisdiction” over the state claims. Also, the Supreme Court will occasionally take appeals from a state supreme court where state law raises an important issue of federal law to be decided. For example, a convict on death row may claim that the state’s chosen method of execution using the injection of drugs is unusually painful and involves “cruel and unusual punishment,” raising an Eighth Amendment issue.

There is also a broad category of cases heard in federal courts that concern only state legal issues—namely, cases that arise between citizens of different states. The federal courts are permitted to hear these cases under their so-called diversity of citizenship jurisdiction (or diversity jurisdiction). A citizen of New Jersey may sue a citizen of New York over a contract dispute in federal court, but if both were citizens of New Jersey, the plaintiff would be limited to the state courts. The Constitution established diversity jurisdiction because it was feared that local courts would be hostile toward people from other states and that they would need separate courts. In 2009, nearly a third of all lawsuits filed in federal court were based on diversity of citizenship. In these cases, the federal courts were applying state law, rather than taking federal question jurisdiction, where federal law provided the basis for the lawsuit or where the United States was a party (as plaintiff or defendant).

Why are there so many diversity cases in federal courts? Defense lawyers believe that there is sometimes a “home-court advantage” for an in-state plaintiff who brings a lawsuit against a nonresident in his local state court. The defense attorney is entitled to ask for removal to a federal court where there is diversity. This fits with the original reason for diversity jurisdiction in the Constitution—the concern that judges in one state court would favor the in-state plaintiff rather than a nonresident defendant. Another reason there are so many diversity cases is that plaintiffs’ attorneys know that removal is common and that it will move the case along faster by filing in federal court to begin with. Some plaintiffs’ attorneys also find advantages in pursuing a lawsuit in federal court. Federal court procedures are often more efficient than state court procedures, so that federal dockets are often less crowded. This means a case will get to trial faster, and many lawyers enjoy the higher status that comes in practicing before the federal bench. In some federal districts, judgments for plaintiffs may be higher, on average, than in the local state court. In short, not only law but also legal strategy factor into the popularity of diversity cases in federal courts.