Military personnel are members of the armed forces. Usually, personnel are divided into military branches roughly defined by certain circumstances of their deployment. Those who serve in a typical large land force are soldiers, making up an army. Those who serve in seagoing forces are seamen or sailors, and their branch is a navy. Marines serve in a marine corps. In the 20th century, the development of powered flight aircraft prompted the development of air forces, serviced by airmen. Designated leaders of military personnel are officers. These include commissioned officers, warrant officers and non-commissioned officers (NCOs). For naval forces, non-commissioned officers are referred to as petty officers (POs).