The Court hears a case when four justices agree to review a lower court decision. This unwritten rule is called “the rule of four.” Often the Court will hear a case when the justices believe that they must settle a conflict in the lower courts—sometimes referred to as a split in the circuits or split in the different state high courts.
The Justices of the 1894 U.S. Supreme Court (from left to right): Horace Gray, Howell E. Jackson, Stephen J. Field, Henry B. Brown, Melville W. Fuller (Chief Justice), George Shiras Jr., John M. Harlan, Edward D. White (who would succeed Fuller as Chief Justice), and David J. Brewer. The number of justices has been set at nine since the Judiciary Act of 1869 (Library of Congress).