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http://usmilitary.about.com/cs/generalinfo/a/military101_3.htm
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Rank/Rate
There are three general categories of rank/rate (Note: The Navy/Coast Guard calls it "rate," the other services refer to it as "rank"): Enlisted personnel, Warrant Officers, and Commissioned Officers.
Enlisted personnel. Enlisted members are the "backbone" of the military. They perform the primary jobs that need to be done. Enlisted members are "specialists." They are trained to perform specific specialties in the military. As enlisted personnel progress up the ranks (there are nine enlisted ranks), they assume more responsibility, and provide direct supervision to their subordinates.
Enlisted personnel in certain grades have special status. In the Army, Air Force, and Marine Corps, this status is known as "Noncommissioned Officer status, or "NCO." In the Navy and Coast Guard, such enlisted are known as "Petty Officers." In the Marine Corps, NCO status begins at the grade of E-4 (Corporal).
In the Army and Air Force, enlisted personnel in the grades of E-5 through E-9 are NCOs. However, some Army E-4s are laterally promoted to "corporal," and are considered NCOs.
Also in the Army and Air Force, personnel in the grades of E-7 to E-9 are known as "Senior NCOs."
In the Marine Corps, those in the grades of E-6 through E-9 are known as "Staff NCOs."
In the Navy/Coast Guard, Petty Officers are those in the grades of E-4 through E-9. Those in the grades of E-7 to E-9 are known as "Chief Petty Officers."
To join the military today, and become an enlisted member, requires a high school diploma (although a very few -- less than 10 percent each year, are accepted with "alternative credentials," such as a GED). However, a majority of enlisted members on active duty today have some college. Many have associates and bachelor degrees. Some even have higher-level degrees, such as masters and doctorates.
Warrant Officers. Warrant Officers are very highly-trained specialists. This is where they differ from commissioned officers. Unlike commissioned officers, warrant officers remain in their primary specialty to provide specialized knowledge, instruction, and leadership to enlisted members and commissioned officers alike.
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