Two cases on federal civil procedure law:
"Swift v. Tyson, 41 U.S. (16 Pet.) 1 (1842), was a case brought in diversity in the Circuit Court for the Southern District of New York on a bill of exchange accepted in New York in which the Supreme Court of the United States determined that United States federal courts that heard cases brought under their diversity jurisdiction under the Judiciary Act of 1789 must apply statutory state laws when the state legislatures in question had spoken on the issue but did not have to apply the state's common law if states legislatures had not spoken on the issue.
The ruling meant that the federal courts that decided matters not specifically addressed by the state legislature had the authority to develop a federal common law."
"Erie Railroad Co. v. Tompkins, 304 U.S. 64 (1938), is a landmark decision by the Supreme Court of the United States in which the Court held that federal courts did not have the judicial power to create general federal common law when hearing state law claims under diversity jurisdiction. In reaching this holding, the Court overturned almost a century of federal civil procedure case law, and established the foundation of what remains the modern law of diversity jurisdiction as it applies to United States federal courts."
(from Wikipedia)