The four-star rank (O-10) of Admiral in the Navy and Coast Guard or General in the Marine Corps is a temporary position reserved for a mere handful of officers from each service: U.S. Code caps the number of USMC Generals and Navy Admirals at two and six, for example. In the Marine Corps, those two slots are assigned to General Officers appointed as Commandant of the Marine Corps and Assistant Commandant. Similarly, four-star Navy Admirals serve in such positions as Chief or Vice Chief of Naval Operations and head of major Command such as Pacific Fleet or Fleet Forces. However, both Marine Corps and Navy General Officers may earn their fourth star as head of a Joint Command without affect the limits set for either service.
Collar grade insignia are worn by officers in these three services on just a handful of uniforms. Admirals in the Navy wear the insignia only on the collar of the short-sleeve shirt of the Service Khaki, while their Coast Guard counterparts are required to wear it with the Winter Dress Blue uniform and when wearing the Service Dress Blue uniform without a jacket. Marine Corps Generals wear collar grade insignia on khaki shirts, as well as on utility uniforms (including the Marine Corps Combat Utility Uniform, or MCCUU).
The same insignia, made with silver starts 9/16” in diameter, is worn on the right side of Garrison Caps issued by all three services.
Placement of stars on uniforms and headgear worn by all General Officers in grade O-7 to O-10 is the responsibility of aides.