In the United States Army, United States Air Force and United States Marine Corps, all ranks of Sergeant are termed NCOs, as are Corporals in the Army and Marine Corps. The rank of Corporal (E-4) in the Army is a junior NCO, and is to be shown the same respect as any other NCO. In the United States Navy and United States Coast Guard, all ranks of Petty Officer are so designated. Junior NCOs (E-4's through E-6 grade) function as first tier supervisors and technical leaders. Of all five branches of service, only the Marine Corps authorizes its NCOs to carry swords on ceremonial occasions.
NCOs serving in the top three enlisted grades (E-7, E-8, and E-9) are termed senior noncommissioned officers (Chief Petty Officers in the Navy and Coast Guard). Senior NCOs are expected to exercise leadership at a more general level. They lead larger groups of service members, mentor junior officers, and advise senior officers on matters pertaining to their areas of responsibility. Within the Marine Corps, senior NCOs are referred to as Staff NCOs and also include the rank of Staff Sergeant (E-6). A select few senior NCOs in paygrade E-9, serve as Senior Enlisted Advisors to senior commanders in each Service (e.g., major command, fleet, force, etc.) and in DoD (unified commands, e.g., STRATCOM, EUCOM, PACOM, etc., and DoD agencies, e.g., DISA, DIA and NSA. One senior E-9, selected by the Service Chief of Staff, is the ranking NCO/PO in that Service, holds the highest enlisted rank for that Service, and is responsible for advising their service Secretary and Chief of Staff. One E-9 holds a similar position as the SEA to the Chairman of the Joint Chiefs of Staff. Senior Enlisted Advisors, Service Enlisted Advisors and the SEA to the Chairman advise senior officer and civilian leaders on all issues affecting operational missions and the readiness, utilization, morale, technical and professional development, and quality of life of the enlisted force.
Unlike Warrant Officers in other militaries, Warrant Officers in the United States Armed Forces are considered specialty officers and fall in between non-commissioned and commissioned officers. Warrant officers also have their own rank tier and paygrade. However, when a Warrant Officer achieves the rank of Chief Warrant Officer, CWO2 or higher, they are commissioned and are considered as commissioned officers just like any other regular commissioned officer but are still held in a different paygrade tier. They are entitled to salutes from their juniors, an officer's sword and uniform, but for much of the UCMJ are considered on par with NCOs.
http://en.wikipedia.org/wiki/Ranks_and_ ... s_enlisted
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