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Lieutenant Commander (Lieutenant-Commander in the Royal Navy)^[1] is a commissioned officer rank in many navies superior to a Lieutenant and subordinate to a Commander. The corresponding rank in most armies, (armed services) and air forces is Major, and in the Royal Air Force and other Commonwealth air forces is Squadron Leader also.

A Lieutenant Commander is a senior department officer on a large ship or shore installation. They may also be commanding officer or executive officer (second-in-command) of a smaller ship or installation.

Lieutenants were commonly put in command of smaller vessels not warranting a Commander or Captain: such a Lieutenant was called a "Lieutenant Commanding" or "Lieutenant Commandant" in the United States Navy, and a "Lieutenant in Command" or "Lieutenant and Commander" in the Royal Navy. The USN settled on "Lieutenant Commander" in 1862, and made it a distinct rank; the RN followed suit in March 1914.

The insignia worn by a Royal Navy Lieutenant-Commander (Lt Cdr) is two medium gold braid stripes with one thin gold stripe running in between, placed upon a navy blue/black background. The top stripe has the ubiquitous loop used in all RN officer rank insignia. The RAF follows this pattern with its equivalent rank of Squadron Leader.

Having fewer officer ranks than the army, the RN previously split some of its ranks by seniority (time in rank) to provide equivalence: hence a Lieutenant with fewer than eight years' seniority wore two stripes, and ranked with an army Captain; a Lieutenant of eight years or more wore two stripes with a thinner one in between, and ranked with a Major. This distinction was abolished when the rank of Lieutenant-Commander was introduced, the new rank taking the insignia and army equivalence of a senior Lieutenant.

Similar insignia is worn by Lieutenant-Commanders in other Commonwealth navies, such as the Royal Australian Navy (which uses the abbreviation "LCDR"), the Royal New Zealand Navy and the Canadian Forces Maritime Command (formerly the Royal Canadian Navy; which uses the abbreviation "LCdr"). Unlike the United States Navy, personnel addressing a Lieutenant-Commander do not abbreviate the rank to "Commander" in the Royal Navy or other Commonwealth Navies.

	Officer ranks of the British Armed Forces
	Student Officer	OF(D)	OF-1		OF-2	OF-3	OF-4	OF-5	OF-6	OF-7	OF-8	OF-9	OF-10
Royal Navy:		Mid	SLt		Lt	Lt Cdr	Cdr	Capt	Cdre	RAdm	VAdm	Adm	Adm of the Fleet
Royal Marines:			2Lt	Lt	Capt	Maj	Lt Col	Col	Brig	Maj Gen	Lt Gen	Gen
Army:	OCdt		2Lt	Lt	Capt	Maj	Lt Col	Col	Brig	Maj Gen	Lt Gen	Gen	FM
Royal Air Force:	OC		APO / Plt Off	Fg Off	Flt Lt	Sqn Ldr	Wg Cdr	Gp Capt	Air Cdre	AVM	Air Mshl	Air Chf Mshl	MRAF

Main article: Lieutenant Commander (United States)

U.S. commissioned officer ranks
	Student Officer	O-1	O-2	O-3	O-4	O-5	O-6	O-7	O-8	O-9	O-10	Special (wartime only)	Special (post war honor only)
Army:	CDT/OC	2LT	1LT	CPT	MAJ	LTC	COL	BG	MG	LTG	GEN	GA	General of the Armies
Marine Corps:	Midn	2ndLt	1stLt	Capt	Maj	LtCol	Col	BGen	MajGen	LtGen	Gen	(no equivalent)	(no equivalent)
Navy:	MIDN/OC	ENS	LTJG	LT	LCDR	CDR	CAPT	RDML	RADM	VADM	ADM	FADM	Admiral of the Navy
Air Force:	Cadet (CDT)/OT	2nd Lt	1st Lt	Capt	Maj	Lt Col	Col	Brig Gen	Maj Gen	Lt Gen	Gen	Gen of the Air Force	(no equivalent)
Coast Guard:	CDT	ENS	LTJG	LT	LCDR	CDR	CAPT	RDML	RADM	VADM	ADM	(no equivalent)	(no equivalent)
PHSCC:	(no equivalent)	ENS	LTJG	LT	LCDR	CDR	CAPT	RDML	RADM	VADM	ADM	(no equivalent)	(no equivalent)
NOAA Corps:	(no equivalent)	ENS	LTJG	LT	LCDR	CDR	CAPT	RDML	RADM	VADM	(no equivalent)	(no equivalent)	(no equivalent)