ABSTRACT :: After 700 years of common-law history and nearly 200 years of constitutional history, the Supreme Court concluded that the constitutionally permissible minimum jury size could not be inferred from the language or the history of the Constitution. The answer, said the Court in Williams v. Florida, could be found only through a “functional analysis” of the performance of smaller juries (that is, empirical examination of the behavior of different-sized juries). The Court implicitly abandoned that analysis in Ballew v. Georgia, when it held that juries with fewer than six members were unconstitutional-a decision based on nothing more than the ipse dixit of the Justices. This Essay sets out the historical and empirical infirmities of the Williams line of cases. It summarizes the jury sizes required in criminal prosecutions throughout the United States; examines the Sixth Amendment history of the jury trial; argues that this history supports the position that the Constitution intended twelve-person juries; reviews Florida’s jury trial history; and summarizes the empirical research undertaken since Williams. This Essay concludes that at present no sound basis exists in law for knowing the minimum size of a constitutionally permissible jury. Williams, having become a dead letter in Ballew, should either be ratified (and the theory of functional equivalence applied conscientiously) or be formally reversed to allow courts either to develop a sound theory of the constitutionality of jury size or to restore the jury to its traditional size.