United States Federal Courts of Appeals have, in each circuit, a large number of judges appointed to them, usually between eleven and twenty. Not all judges sit on each appeal. Instead a panel of three judges initially hears the case (in a procedure that is usually as-of-right (See Appeal-as-of-Right) or relatively automatic). After the panel rules, the parties may petition for a hearing by the entire court of appeal (or in some very large courts such as the 9th circuit a selection of eleven judges (one the chief judge)), sitting en banc. However, such an en banc hearing is not automatic. Rather, under the rules of the appellate court, a number of judges, usually just less than a majority of the total number on that appellate court must review the petition and decide that it justifies such a rehearing. For this reason, unanimous rulings of a panel are unlikely to secure an en banc rehearing.
Amicus briefs are often filed in support of petitions for rehearing en banc. Such re-hearings are rare. For example, between September 1999 and 2000 the regional federal appeals courts heard 27,516 appeals of which only seventy-three were decided en banc. Although en banc re-hearings by the United States Court of Appeals for the Federal Circuit are more common, they are still comparatively rare.