The states are independent of each other, but, since they are not capable of negotiating with each other, or having foreign relations with reference to each other, it is provided that controversies between them may be determined in the federal courts. Such controversies have usually been as to boundaries.

As will be noticed in the next section, a state may not be sued, even in a federal court, by its own citizens or the citizens of another state or of a foreign government for the purpose of compelling it to pay its debts; but if one state has a claim for money against another, the controversy relating to such claim is a controversy between states, bringing it within the scope of the federal jurisdiction. Thus, where one state was the owner in its own right of bonds of another state, it was held that suit on such bonds could be maintained by the one state against the other in the federal courts (South Dakota v. North Carolina).