Complete diversity rule

Complete diversity means that the citizenship of every plaintiff has to differ from the citizenship of every defendant. If even one plaintiff and one defendant share a state of citizenship, federal diversity jurisdiction is destroyed. This rule comes from an old Supreme Court decision called Strawbridge v. Curtiss and still controls today. It often forces plaintiffs to be careful about whom they sue and where, and it gives defendants a way to challenge a federal filing they would rather see in state court. Companies are treated as citizens of both their state of incorporation and the state of their main headquarters for this purpose.