This Article aims to clarify the content of the void-for-vagueness doctrine and defend its historical pedigree by drawing attention to a fundamental aspect of the Supreme Court's vagueness decisions-that vagueness analysis significantly de-pends on whether the law at issue is a federal or state law. That simple distinction has considerable explanatory power. It reveals that the doctrine emerged in the late nineteenth century in response to two simultaneous changes in the legal landscape- first, the availability of Supreme Court due process review of state penal statutes under the Fourteenth Amendment, and second, a significant shift in how state courts construed those statutes. The federal-state distinction also divides the Court's deci-sions into two groups with mostly separate concerns. It reveals that separation-of -powers concerns primarily motivate the Court's vagueness decisions involving fed-eral laws, while federalism concerns are the driving force in its vagueness decisions involving state laws. In the vast majority of cases involving a federal law, the Court will narrowly construe the law to avoid vagueness concerns. In cases involving a state law, by contrast, the Court will follow any preexisting state-court construction of the law, however indefinite it may be, with the result that vagueness analysis amounts to a due process limitation on judicial construction. Proper recognition of the federal-state distinction would result in fewer vagueness cases that reach the Supreme Court and more penal laws that are narrowly construed. That would pro-mote the rule of law by increasing the precision of criminal laws and reducing the risk of arbitrary enforcement-the very goals the vagueness doctrine seeks to achieve.
