One of these requirements [of a proper jury verdict] was unanimity. Wherever we might look to determine what the term “trial by an impartial jury trial” meant at the time of the Sixth Amendment’s adoption—whether it’s the common law, state practices in the founding era, or opinions and treatises written soon afterward—the answer is unmistakable. A jury must reach a unanimous verdict in order to convict. The requirement of juror unanimity emerged in 14th century England and was soon accepted as a vital right protected by the common law. As Blackstone explained, no person could be found guilty of a serious crime unless “the truth of every accusation . . . should . . . be confirmed by the unanimous suffrage of twelve of his equals and neighbors, indifferently chosen, and superior to all suspicion.” A “‘verdict, taken from eleven, was no verdict’” at all.
This same rule applied in the young American States. Six State Constitutions explicitly required unanimity. Another four preserved the right to a jury trial in more general terms. But the variations did not matter much; consistent with the common law, state courts appeared to regard unanimity as an essential feature of the jury trial. It was against this backdrop that James Madison drafted and the States ratified the Sixth Amendment in 1791. By that time, unanimous verdicts had been required for about 400 years.