Two years ago in United States v. Windsor, the Supreme Court held that the Constitution forbids the federal government from treating same-sex marriages differently than opposite-sex marriages. The Court’s majority, writing through Justice Anthony Kennedy, recognized that discriminating against lawfully wed same-sex couples violates the Fifth Amendment’s “prohibition against denying to any person the equal protection of the laws,” a prohibition which the “equal protection guarantee of the Fourteenth Amendment makes … all the more specific and all the better understood and preserved.” Because Windsor challenged only the federal Defense of Marriage Act, no state marriage laws were directly affected. Given the Court’s broad language, however—especially its direct reference to the Equal Protection Clause—many (including Justice Antonin Scalia in dissent) predicted that the opinion’s reasoning would eventually be used to strike down state laws and constitutional provisions that deny marriage licenses to same-sex couples. They were right. Windsor served as the basis for dozens of challenges to marriage laws across the country, initially resulting in an unbroken series of victories for marriage equality in federal courts, which the Supreme Court declined to review. Finally, the U.S. Court of Appeals for the Sixth Circuit (which covers Kentucky, Michigan, Ohio, and Tennessee) issued a contrary ruling, holding that “the people who adopted the Fourteenth Amendment understood it to require the States to change the definition of marriage.” The Supreme Court could thus no longer delay taking up the marriage debate. Cato has accordingly filed what will almost certainly be our final brief on this issue. Joining with noted originalist scholar (and Federalist Society co-founder) Steven Calabresi and Yale law professor William Eskridge—one of the leading experts on American legal history—we urge the Court to reverse the Sixth Circuit’s decision and finally fulfill the Constitution’s promise of equal protection under law to millions of gay Americans and their children. We argue that the lower court’s ruling was inconsistent with the original meaning of the Fourteenth Amendment’s Equal Protection Clause. The fact that the provision’s ratifiers didn’t automatically or explicitly understand that it would eventually require states to recognize same-sex marriages is irrelevant; all that matters is what it meant in 1868 for a state to “deny to any person within its jurisdiction the equal protection of the laws.” As our brief shows, this language was based on similar language in state constitutions and was widely (and properly) understood as prohibiting the states from passing what’s known as “caste” legislation—laws that create “second-class” citizens with inferior legal rights. While some jurists and academics have argued that the Equal Protection Clause only prohibits discrimination on the basis of race—and not, say, gender, religion, national origin, or sexual orientation—this is an ahistorical view that ignores mountains of evidence to the contrary. The framers of the Fourteenth Amendment explicitly rejected earlier versions of the clause that would have restricted it to prohibiting race-based discrimination. Instead, they adopted language that “establishes equality before the law, and … gives to the humblest, the poorest, and most despised … the same rights and the same protection before the law as it gives to the most powerful, the most wealthy, or the most haughty” and “abolishes all class legislation in the States and does away with the injustice of subjecting one caste of persons to a code not applicable to another.” Essentially, the Equal Protection Clause means, in 1868 as in 2015, exactly what it says: states cannot have one set of laws for the rich and another for the poor, separate schools for white and black students, or marriage licenses only for opposite-sex couples.