When the Supreme Court opens its 2026-27 term, the justices will again rely on a 100-year-old custom to decide which petitions to hear. The rule of four—agreed to by Chief Justice William Howard Taft during the 1925 Judiciary Act negotiations—allows four justices to grant certiorari even if a majority opposes review. Though not written in the Constitution, federal statute, or the court's own rules, it has shaped the docket for a century, turning down roughly 4,000 petitions each term while hearing only 55–60 cases.
The rule's informal nature creates wrinkles. For emergency stays, five votes are needed, so a case can be granted review while the underlying action proceeds—a scenario that proved fatal in a 1990 Texas execution. To prevent such outcomes, justices sometimes offer a 'courtesy fifth vote' for a stay, as seen in 2016 when Chief Justice John Roberts and Justice Stephen Breyer each provided the extra vote. Similarly, dismissing a case as improvidently granted (a DIG) may require six votes, including one from the original four, to avoid undermining the rule.
Justice John Paul Stevens questioned the rule in 1983, arguing it led to unnecessary cases when five justices opposed review, but no change followed. The rule persists partly for its populist appeal: it allows a minority to bring important issues before the court, making the institution more accessible. As the justices return to conference, this unwritten practice remains central to their work, a reminder that custom often carries as much weight as law in the Supreme Court's operations.