The composition of the corps areas divided the United States geographically by state lines and population density into nine multi-state area commands roughly equal in population. Each corps area was responsible for organizing two tactical corps consisting of three infantry divisions each. Each corps area also had responsibility for organizing various other field army, General Headquarters Reserve, Zone of the Interior (later designated as Corps Area Service Command), and Communications Zone units. The First, Second, Third, Fourth, Eighth, and Ninth Corps Areas also organized units to man various fixed coastal defenses. The corps areas were further grouped into three army areas of two field armies each.