What is the 'rule of four' in the U.S. Supreme Court?

The Supreme Court’s rule of four means that at least four justices must agree to grant a petition for certiorari and hear the case.

Certiorari is the Court’s discretionary procedure for selecting most of the cases it reviews. Thousands of petitions may be filed in a term, but only a small fraction receive review. Four votes are enough to place a case on the Court’s merits docket, even though the Court has nine justices and four is not a majority.

The practice helps prevent five justices from completely controlling the Court’s agenda. It is a longstanding custom rather than a requirement expressly written in the Constitution, a federal statute, or the Court’s published rules.

The rule concerns accepting a case, not deciding its eventual winner. A four-justice vote to grant certiorari does not mean four justices support one side’s legal position. The Court generally announces grants and denials of review without identifying how individual justices voted.