Recent debates about popular constitutionalism and judicial supremacy have focused on the question of who interprets the Constitution. This article reframes the debate by asking what legal sources courts apply to protect individual rights from government infringement. Throughout the nineteenth century, federal courts applied a mix of international law, statutes and common law to protect fundamental rights and restrain government action. This article uncovers the forgotten history of nineteenth century public law litigation.Professors Post and Siegel have advocated “policentric constitutional interpretation,” wherein the Supreme Court shares authority for constitutional interpretation with other actors. By analogy, this article introduces th...