Military personnel is a blanket term used to refer to members of 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 twentieth century, the development of powered flight prompted the development of air forces, serviced by airmen. Their designated leaders are Commissioned Officers, called officers for all types of forces, & Non-Commissioned Officers (NCO's) for land/air forces & Petty Officers (PO's) for naval forces .