The etymology of the rank "lieutenant commander" comes from the early 18th century when senior lieutenants of the United States Navy were given command of smaller support vessels and referred to as the "lieutenant commanding" the vessel. This rank name became official in both the United States Navy and the British Navy by the late 1870s as "lieutenant commander." It survives today among the naval-driven traditions of the Federation's Starfleet.
A lieutenant commander is a mid-level line officer rank, a grade higher than a lieutenant and a grade below commander in most rank structures, although some organizations may differ or omit this rank. The lieutenant commander rank is roughly equivalent with the authority of a major of military forces.
In the Romulan Star Navy, the rank of subcommander (Romulan language erei'riov) is roughly equivalent to a Starfleet lieutenant commander, although Romulan subcommanders are often first officers and fill other positions with standings approximate of Starfleet commanders. (TOS novel: The Romulan Way; TOS episode: "The Enterprise Incident")
In the Cardassian language, dalin is a Cardassian officer rank title which, in translation to Federation Standard, is approximately equivalent to a lieutenant commander. (TLE - Terok Nor novel: Day of the Vipers)
The common title of address for a lieutenant commander is "commander," when the generic "sir" is otherwise inappropriate. This usage is similar to how a lieutenant colonel of a military unit is addressed simply as "colonel".
A lieutenant commander in Starfleet may be assigned as a department head or senior staff member of a larger command such as a capital ship or starbase. Some examples include Chief Engineer Montgomery Scott and Chief Medical Officer Leonard McCoy of the USS Enterprise, Operations Manager Data and Chief Engineer Geordi La Forge of the USS Enterprise-D, Tactical Officer Tuvok of the USS Voyager or Chief Science Officer Jadzia Dax of Starbase Deep Space 9. (TOS; TNG; VOY; DS9)
A lieutenant commander could also serve as the commanding officer of a smaller starship or the first officer of a mid-size vessel. Jadzia Dax commanding the USS Defiant and First Officer Chakotay of Voyager filled these roles, respectively. On larger vessels, a lieutenant commander could also fill a role as second officer or third officer, such as the aforementioned positions of Scotty, Data, Tuvok and Dax on their respective assignments. (DS9, VOY, TOS, TNG)
In the Klingon Defense Force, the rank of lieutenant commander is represented by a rank insignia of a stylized, tri-segmented, "z" shape, within a circle. (FASA RPG module: The Klingons: Star Fleet Intelligence Manual)
|Enlisted ranks||Commissioned ranks||Flag ranks|
|Earth SF||CREW|| PO2
|Starfleet & SFMC|| ENS JG
||LT JG||LT||LT CMDR||CDR|| CDORE
||2ND LT||1ST LT||CAPT||MAJOR||LT COL||COL||BRIG||GEN|
| Klingon Defense Force|
naval & marine
|ENL (bekk)||PO/LDR (DevwI')
||ENS (lagh)||LT JG||LT (Sogh)||LT CMDR||CMDR (la')||CAPT (HoD)||CDORE (totlh)||ADM ('aj)||FADM|
| ENL (mang) 2C
||CPL (Da')|| SGT (bu')
||LT JG||LT (Sogh)||CAPT||MAJ||LT COL||COL||BRIG ('ech)||MAJ GEN||GEN (Sa')|
|UHLAN||DECURION||ANTE CENTURION (erein)||SUBLT||SUBCMDR (erei'riov)||CMDR (rh'iov)||CMDR GEN (khre'riov)||GENERAL (Enriov)|
|SR CENTURION (enarrain)||ADMIRAL|
|EQUITORIAM||SECONDAM||TRINAM||CENTURION (Arrain)||LEGATE||FIELD PRIMUS||PRIMUS||GRAND PRIMUS|