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Why can Congress change the number of justices on the Supreme Court?

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A person ascends the steps of the west facade of the United States Supreme Court Building in Washington, D.C., with its tall columns and a round fountain in the foreground. — Supreme Court of the United States The west facade of the U.S. Supreme Court building, Washington, D.C.

Congress has the authority to change the number of justices on the Supreme Court of the United States because the U.S. Constitution does not specify the size of the Court, instead placing the power to determine that number directly in the hands of Congress.12

Because the Constitution is silent on the matter, the size of the Court has been established and adjusted through legislative acts throughout American history. The number of justices has fluctuated between five and ten since the first Judiciary Act of 1789, which originally set the number at six.31

While these changes have sometimes been driven by the need to manage the Court's workload, the size of the bench has also been subject to political manipulation. For example, in 1866, Congress reduced the number of seats to prevent President Andrew Johnson from making appointments, and in 1937, President Franklin D. Roosevelt proposed a "court-packing" plan to add justices who would be more favorable to his New Deal policies. The number of justices has remained at nine since 1869.321