Why twelve? Why not 14 or 10? It’s not an unfair question but for the fact that a number existed at the time the Bill of Rights was ratified that was so deeply embedded in common law history and tradition that it was accepted as the number of people that made up a jury for the trial of serious criminal offenses.
And while scholars may debate the precise moment when the common-law jury came to be fixed at 12 members, this much is certain: By the time of the Sixth Amendment’s adoption, the 12-person criminal jury was “an institution with a nearly four-hundred-year-old tradition in England.”
