The Navy’s current system of Admirals was not clearly defined until well into the 20th century, when U.S. Code was crafted to determine the number of different ranks of Admirals that could serve at any given time.
When Congress created the rank of Rear Admiral in 1862, the insignia consisted of two stars, and until World War II and the creation of a temporary rank of Commodore there were no one-star Admirals. Although there were over 100 Navy officers holding the rank of Commodore at the end of the war, promotions to Rear Admiral were not common.
The Commodore rank appeared again the late 1970s and early 1980s, but inter-service complaints over the Navy’s practice of allowing officers at a paygrade of 0-7 (one-star) to wear the insignia of a Rear Admiral (paygrade O-8). Much ink was spilled and confusion generated over this policy, and in 1984 the current system was established.
The Rear Admiral (lower half) is a one-star Admiral with a paygrade of 0-7 and the designation RDML. Real Admiral is indicated with two stars and has a paygrade of O-8; three stars indicate a Vice Admiral and a paygrade of O-9; and a full Admiral wears four stars and has a paygrade of O-10.
Five-star Admirals are Fleet Admirals, and there have been only four Admirals to hold the rank: William Leahy, Chester Nimitz, William Halsey, and Ernest King. The rank was created through an Act of Congress in 1944, which specified it would be held by only four Admirals at any given time, and that the provisions of the Act would no longer be effective six months after the end of the war. No legislation has been signed to authorize this rank since that time.