Article III of the United States Constitution leaves it to Congress to fix the number of justices. The Judiciary Act of 1789 called for the appointment of six justices, and as the nation's boundaries grew, Congress added justices to correspond with the growing number of judicial circuits: seven in 1807, nine in 1837, and ten in 1863.

In 1866, at the behest of Chief Justice Chase, Congress passed an act providing that the next three justices to retire would not be replaced, which would thin the bench to seven justices by attrition. Consequently, one seat was removed in 1866 and a second in 1867. In 1869, however, the Circuit Judges Act returned the number of justices to nine,[64] where it has since remained.