In the Thompson v. Utah case of 1897, the U.S Supreme Court ruled that a defendant has a constitutional right to be tried by a jury composed of twelve persons, as was common law practice at the time when the Sixth Amendment was adopted. The case involved John Henri Thompson who had been convicted in Utah for second-degree murder by an eight-person jury instead of twelve. He appealed his conviction on grounds that it violated his Sixth Amendment rights which guarantees trial "by an impartial jury". The court agreed with him and held that since he committed his crime before Utah became a state (when federal laws still applied), he should have been tried under those laws which required juries to consist of twelve people not eight as per new state law. Thus, this ruling established precedent regarding size and composition requirements for juries in criminal trials.

In the dissenting opinion for Thompson v. Utah, Justice Harlan disagreed with the majority's interpretation of the Constitution and its application to territories acquired by the United States. He argued that when a territory becomes part of the U.S., it is subject to all laws and regulations established by Congress, including those regarding jury trials. In his view, this includes changes in how juries are composed or function as long as they still uphold principles of fairness and justice. Therefore, he believed that Utah was within its rights to reduce jury sizes from twelve members to eight after becoming a state without violating constitutional protections for defendants like Thompson who were charged while Utah was still a territory but tried after it achieved statehood.