While Article III describes the judicial branch, stating that the "judicial Power of the United States, shall be vested in one supreme Court, and in such inferior Courts as the Congress may from time to time ordain and establish," the U.S. Constitution does not specify the number of Justices who sit on the Supreme Court or their duties.

The Judiciary Act of 1789 assigned a chief justice and five associate justices to the original Supreme Court.  Congress decreased the number of associate justices to four in 1801, but increased the number to six in 1807.  The number of associate justices changed again to eight in 1837, nine in 1863, and then was reduced to six in 1866.  An act of 1869 provided for a chief justice and eight associate justices, which has been the size of the Supreme Court ever since.