<P> In the United States, commander is a military rank that is also sometimes used as a military billet title, depending on the branch of service . It is also used as a rank or title in some organizations outside the military, particularly in police and law enforcement . </P> <P> The commander rank started out as "Master and Commander" in 1674 within the British Navy for the officer responsible for sailing a ship under the Captain and some times second - in - command . Sub-captain, under - captain, rector and master - commanding was also used for the same position . With the Master and Commander also serving as captain of smaller ships, the British Navy subsumed as the third and lowest of three grades of captain given the various sizes of ships . The American Continental Navy adopted the tri-graded captain ranks . Captain 2nd Grade, or Master Commandant, became Commander in 1838 . </P> <P> In the United States Navy, the United States Coast Guard, the United States Public Health Service Commissioned Corps, and the National Oceanic and Atmospheric Administration Commissioned Officer Corps, commander (abbreviated "CDR") is a mid-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 services are entitled to embellishment at O - 4 rank . Promotion to commander in the US 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 . </P> <P> A commander in the U.S. 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, or informally referred to as "skipper". Commanding officers of aviation squadrons and shore activities may also be informally referred to as "skipper" but never as "captain ." </P>

What does cdr stand for in the navy