Lieutenant commander
Lieutenant commander was a commissioned officer rank, the equivalent of which was used by the service organizations of many civilizations. It was common in informal circumstances to refer to lieutenant commanders simply as "commander" by both those under their command and their commanding officers. (, et al.)

History 
A traditional naval rank, lieutenant commander was once used in the old navies of Earth, among them the Royal Navy and the United States Navy. In these organizations, a lieutenant commander was a grade between the lieutenant and commander grades with the rank equivalent to a major in the marine or ground forces. Lieutenant commanders normally served as senior department heads, executive officers on smaller vessels, and in some cases as starship commanders. Another position that could be held by a lieutenant commander was as an instructor for Starfleet's Advanced Tactical Training. ()

The rank of lieutenant commander was also known to have existed in parallel universes. In the mirror universe, the Terran Empire  was using this rank in 2155 (with  being an example) and the rank was still being used in 2267. (; )

The origins of the service rank of lieutenant commander in Starfleet dated to the mid-2160s. () 

During the 2260s, the insignia was at first the same as that of commander, but by 2266 had developed into its own design; where previously, certain lieutenant commanders wore the full braid insignia of a commander while still retaining their rank of lieutenant commander. ()

In 2267, Lieutenant Commander Spock served as the executive officer of the . ()

In 2269, Lieutenant Commanders Tom Markel served as the commanding officer of the . ()

During the early 2360s, Captain Leyton promoted Benjamin Sisko to lieutenant commander when he gave him the post of executive officer aboard the ; he, thereafter, taught Sisko everything he knew about being a leader. () 

Around the same time, Lieutenant Commander William T. Riker served as first officer of the , before being promoted to commander, and first officer of the . ()

When Commander Riker first met Lieutenant Commander Data in 2364, Riker inquired his rank of lieutenant commander was honoray, which Data explained was not, and that he was a member of the Starfleet Class of '78. ()

In 2365, Lieutenant Commander Hester Dealt acted as the medical trustee for the Federation Medical Collection Station. ()

Later that year, during Data's introduction by The Comic to the holodeck crowd at Charnock's Comedy Cabaret, he was described as "the funny man of the stars, the android of antics, that lieutenant commander of mirth." ()

Guinan revealed to Lieutenant Worf, in late 2366, that she had a bet with Captain Jean-Luc Picard that she could make the Klingon officer laugh before he became lieutenant commander. ()

In 2367, Lieutenant Commander Data was given temporary command of the , with his first officer being Lieutenant Commander Christopher Hobson. ()

Regarding the service record of Ensign Ro Laren, as of 2368, Jean-Luc Picard explained that "if she hadn't lost her rank in that incident at Garon IV, she'd be lieutenant commander by now." ()

In 2370, Lieutenant Commander Piersall served as the commanding officer of the . ()

The same year, Lieutenant Commander Calvin Hudson, served Starfleet's attaché to the Federation colonies in the Cardassian DMZ. ()

During the mock sentencing staged for the promotion of Worf to lieutenant commander in 2371, Commander Riker read: "We, the officers and crew of the Enterprise-D, being of sound mind and judgment, hereby make the following charges against Lieutenant Worf. One. That he did knowingly and willfully perform above and beyond the call of duty on countless occasions. Two. Most seriously, that he has earned the admiration and respect of the entire crew." Following the reading of the "charges", Captain Picard granted Worf his new rank, "with all the rights and privileges thereto." ()

In 2374, Lieutenant Commander Jadzia Dax served as the commanding officer of the  during the height of the Dominion War, because she had acted as Captain Sisko's second in command and was the most qualified to take over for him when he had been reassigned to administrative duties at Starbase 375. () However, her posting was temporary until the Captain returned to command a taskforce during Operation Return. () 

The same year, as Captain Kathryn Janeway worked up to the promotion of Lieutenant Tuvok, while aboard , she shared that "[o]ver the past nine years I've come to rely on his insightful and unfailingly logical advice. For outstanding services, chief tactical and security officer, it's my pleasure to grant you the rank of lieutenant commander." Upon receiving his pip, he noted, "I accept it with gratitude and will honor the responsibility that comes with it." () 

Later yet, the same year, Ensign Nog was given a field promotion to lieutenant commander, and assigned as the chief engineer of the . ()

When Lieutenant Commander Greta Vanderweg, a science officer aboard Deep Space 9, was murdered in 2375, it was noted by Odo that she had had twelve years of service in Starfleet. () 

When Ensign Harry Kim pressed Captain Janeway for his first command opportunity of the Kraylor starship Medical Transport 136 in 2377, he pleaded, "I've been on Voyager for almost seven years and I'm still an ensign," further explaining, that in that time, "if we were back home, I'd be a lieutenant by now. Maybe even a lieutenant commander." ()

List of lieutenant commanders 
 List of Starfleet lieutenant commanders

Appendices

Background information 
The earliest Starfleet officer referenced with this rank (in-universe) was Hoshi Sato, as established in her personnel file in , though the insignia is never seen on the . A  article on the 2151 uniforms and rank did not mention lieutenant junior grade or lieutenant commander. () 

During pre-production, Malcolm Reed was going to be a lieutenant commander, though ended up with the rank of lieutenant.

 depicted an inaccurate lieutenant commander insignia for the , with a vertical gold bar split horizontally (instead of one horizontal gold bar). 

Contrary to traditional rank protocol, there have been several clear instances in Star Trek where a lieutenant commander was verbally addressed as "lieutenant":
 In the  premiere , Commander Riker appeared to address Deanna Troi as "Lieutenant", when she was a Lieutenant Commander. 
 On three occasions, Lieutenant Commander Geordi La Forge was referred to as "Lieutenant":
 Once by Tasha Yar in the  third season episode ;
 Once by Captain Picard, later the same season, in the ;
 Once by Data in the fifth season episode . 
 Lieutenant Commander Elizabeth Shelby, who was called a Lieutenant in a briefing by Rear Admiral J.P. Hanson. 
In the alternate future depicted in the Voyager third season episode, , Tom Paris is referred to as Lieutenant by The Doctor despite him wearing three pips.
 In the Enterprise fourth season episode , Commander Tucker refers to his replacement Kelby as lieutenant despite him wearing three pips.
 While addressed as such in reference, not in person, in 2009's ,  wears Lieutenant Commander rank braids, but is referred to by  as "Lieutenant George Kirk" during 's  in 2258.
 Lt. Commander Steve Stevens was addressed by D'Vana Tendi as lieutenant in .
 Lt. Commander  was addressed by Carol Freeman as lieutenant in .

Likewise, Tuvok was also incorrectly identified as lieutenant after his promotion to lieutenant commander during his travels in the Delta Quadrant. Conversely, he was frequently seen wearing lieutenant commander's pips during the first season despite holding the rank of lieutenant at the time; this appears to have been a costume error or possibly a sign that the decision to make him a lieutenant was made after shooting started. 

In another apparent costuming error, Commander Chakotay wore what the  identified as the provisional insignia for a lieutenant commander for all seven seasons, but was never addressed as such, only as a commander.

On a similar note, apparent costuming errors throughout  had Valeris wear lieutenant commander insignia (as well as mismatched uniform department colors), although the intention was to have her just hold the rank of lieutenant. Because it was deemed too expensive to go back and re-shoot the sequences with Valeris wearing the correct rank and uniform color, it was decided to just let it be.

External links 
 
 

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Military ranks