Basic training is followed by a specialized or advanced training phase that provides recruits with a specific area of expertise or concentration. In the Army and Marines, this area is called a military occupational specialty; in the Navy it is known as a rate; and in the Air Force it is simply called an Air Force specialty. 

ARMY -- * For rank and precedence within the Army, specialist ranks immediately below corporal. Among the services, however, rank and precedence are determined by pay grade. 

NAVY/SPACE COMMAND/COAST GUARD -- *A specialty mark in the center of a rating badge indicates the wearer's particular rating. ** Gold stripes indicate 12 or more years of good conduct. *** 1. Master chief petty officer of the Navy and fleet and force master chief petty officers. 2. Command master chief petty officers wear silver stars. 3. Master chief petty officers wear silver stars and silver specialty rating marks. 

Details of rates, ratings, and specialities for Navy
 
Note that the rank insignia shown for the Marines include the subdued and gold rank pins.

On ocassion when E-9s abound, the Officer in Command may choose the senior most E-9 and designate him "The Gunny" or "Top", just as a Navy or Space Command Commander may choose a "Master Chief of the Ship".  This is an honorary, however enlisted and NCOs clearly recognized this special title and act accordingly.  Sometimes the "Top Sergeant", "Master Chief", or "Master Gunny"  at Headquarters will be called "Top", "Chief's Chief", or "Chief Master Gunny" as well.  Again there are no such official ranks.  Somehow, they do tend to appear in fitness reports, however.  

Note also that the Marine Sergeant Major is a higher rank than a Master Gunnery Sergeant even though they are both at the same pay scale, E-9.  Typically the Master Gunny works for the Sergeant Major.  Not all postings have a Sergeant Major "on board".  The difference between the two Senior NCO tracks is defined in the MARCORPPROMMAN publication MCO P1400.32C W.  The First Sergeant and Sergeant Major are administrative and command track, while the Master Sergeant and Master Gunnery Sergeant are on the technical  experts command track.  All four are looked on as key advisors to their commanders on enlisted matters and serve to provide guidance on tradition, lore, experience, and in the case of the Master Sergeant and Master Gunnery Sergeants, techincal expertise in their MOS or occupational speciality.

A third E-9 element is the senior enlisted person of each service. The Sergeant Major of the Army (SMA), the Sergeant Major of the Marine Corps (SGTMAJMC),  the Master Chief Petty Officer of the Navy (MCPON), and the Chief Master Sergeant of the Air Force (CMSAF) are the spokespersons of the enlisted force at the highest levels of their services.  They typically work closely with the Chief of Staff (four star General or Admiral) for their service on all matters pertaining to enlisted personnel.  While these remarkable individuals are no longer in the chain of command, it would be rare for anyone, including most junior officers, not to heed the advice or indeed take an order from the top sergeant of the service.

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