Navy personnel serving as aides to Rear Admirals or to Major and Brigadier Generals in other branches of the military wear Service aiguillettes made of two loops of golden or gilt-covered cord; the same type of Service aiguillette is worn by Naval aides to a Governor of a State or a territory.
While the three- and four-loop aiguillettes correspond directly to the rank of the officer or official whom the Naval aide is reporting to—an aide to a three-star Vice Admiral, for example, wears a Service aiguillette made of three loops—this system is truncated when it comes to aides serving Rear Admirals. That’s because the rank of Rear Admiral actually refers to two grades of flag officers: the one-star Rear Admiral (lower half), and the two-star Rear Admiral (upper half).
The two ranks are distinguished from each other not only by pay grades (0-7 and 0-8), but also abbreviations: the one-star rank is RDML and the two-star rank is RADM.
The rank title of Rear Admiral had its origins in the way in which English fleet commanders delegated authority to sub-commanders. Based upon the Latin word “vice” meaning someone who stands in the place of another, a Vice Admiral is an Admiral’s deputy. In the British Navy, Vice Admirals also had their own deputies—but the phrase “Vice-Vice Admiral” was clunky and confusing. Instead, sub-commanders reporting to the Vice Admiral were called “Rear Admirals” because they were often aboard ships at the rear of the fleet.
Interestingly, while the rank of Rear Admiral was created in the British Navy as subordinate to that of Vice Admiral or Admiral, it was the highest rank in the U.S. Navy when it was established by Congress in 1862 and awarded to David Farragut for his capture of the city of New Orleans.