In the United States, commander is a military rank that is also sometimes used as a military billet title—the designation of someone who manages living quarters or a base—depending on the branch of service. It is also used as a rank or title in non-military organizations; particularly in law enforcement. As rank History The commander rank started out as "Master and Commander" in 1674 within the Royal Navy for the officer responsible for sailing a ship under the Captain and sometimes second-in-command. Sub-captain, under-captain, rector and master-commanding were also used for the same position. With the Master and Commander also serving as captain of smaller ships the Royal Navy subsumed as the third and lowest of three grades of captain given the various sizes of ships. The Continental Navy had the tri-graded captain ranks. Captain 2nd Grade, or Master Commandant, became Commander in 1838. Naval In the Navy, the Coast Guard, the NOAA Corps, and the Public Health Service Corps, commander (abbreviated "CDR") is a senior-grade officer rank, with the pay grade of O-5. Commander ranks above lieutenant commander (O-4) and below captain (O-6). Commander is equivalent to the rank of lieutenant colonel in the other uniformed services. Notably, it is the first rank at which the holder wears an embellished cap whereas officers of the other military services are entitled to embellishment of similar headgear at O-4 rank. Promotion to commander in the Navy is governed by Department of Defense policies derived from the Defense Officer Personnel Management Act (DOPMA) of 1980 or its companion Reserve Officer Personnel Management Act (ROPMA). DOPMA/ROPMA guidelines suggest that 70% of lieutenant commanders should be promoted to commander after serving a minimum of three years at their present rank and after attaining 15–17 years of cumulative commissioned service, although this percentage may vary and be appreciably less for certain officer designators (i.e., primary "specialties") dependent on defense budgets, force structure and needs of the service. A commander in the Navy may command a frigate, destroyer, submarine, aviation squadron or small shore activity, or may serve on a staff afloat or ashore (typically as an action officer or as an executive officer to a flag officer or general officer), or a larger vessel afloat (as either a department head or executive officer). An officer in the rank of commander who commands a vessel may also be referred to as "captain" as a courtesy title,