The chairman of the Joint Chiefs of Staff is, by law, the highest-ranking military officer of the United States Armed Forces, and the principal military adviser to the president of the United States. He leads the meetings and coordinates the efforts of the Joint Chiefs of Staff, comprising the chairman, the vice chairman of the Joint Chiefs of Staff, the chief of staff of the United States Army, the commandant of the United States Marine Corps, the chief of naval operations, the chief of staff of the United States Air Force, the Chief of Space Operations (statutory member after December 2020), and the chief of the National Guard Bureau. The Joint Chiefs of Staff have offices in the Pentagon. The chairman outranks all service chiefs, but does not maintain authority over them, their branches or the Unified Combatant Commands. All combatant commanders receive their orders directly from the secretary of defense.