Commander
Commander was a commissioned officer rank used by the service organizations of many civilizations. 

As a traditional naval grade in Human usage, commander was the second most senior line officer rank, falling between a lieutenant commander and a captain. In Romulan usage, it falls between subcommander and admiral.

During the 20th century on Earth, the United States Navy used this rank. The United States Navy used the insignia of three solid sleeve stripes to denote the rank of commander. The watch station of command duty officer was a position that could be held by a commander. ()

Responsibilities

Starfleet 
The rank of commander was a senior rank, common among executive officers and senior department heads, or even commanders of space stations and other such installations. (; ; DS9 Season 1-DS9 Season 3) 

While the rank was not required to be Chief Medical Officer, the Bridge Officer's Test was required to hold the rank. ()

Other organizations 
In the Xindi-Reptilian military of the 22nd century, some commanders controlled a squadron and a regiment. ()

The rank of commander in the Romulan Star Empire was considered the equivalent of a Starfleet captain. () Also, a sub-commander grade existed in the Romulan military, with this rank also used in the 22nd century by the Vulcan High Command. (; )

The 23rd century Imperial Klingon Fleet used the rank of commander to denote the commanders of smaller space vessels, such as a Bird-of-Prey. () A Klingon rank of captain was used for larger vessels with this title carrying over to the 24th century Klingon Defense Force. (; )

Noted commanders 
Andorian Imperial Guard
Thy'lek Shran
Benthan Guard
Avik
Dramian
 Demos
Federation Starfleet

First Federation
 Balok
Kelvan Empire
 Rojan
Klingon Defense Force
Kang
Kaybok
K'Nera
Kolana
Kor
Kruge
Kuri
Kurn
Morag
Sorval
Nakan military
Saavdra
Romulan Star Empire
Donatra
Konsab
Rekar
Sela
Sirol
Suran
Tebok
Tomalak
Toreth
Suliban Cabal
Raan
Talaxian
Paxim
Tholian Assembly
Loskene
Xindi
Dolim
Kolo
Zalkonian
Sunad

Appendices

See also 
 Field commander
 Honorary commander
 Space Commander

External link 
 

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Military ranks
Titles