Starbase 12 was a Federation starbase administered by Starfleet. This command base was in Sector 25712. It was located on a planet in the Gamma 400 system which was located close to the planets Achrady VII, Ceti Alpha V, Gemaris V, Mab-Bu VI, Pollux IV, and Risa. (TOS: "Space Seed", "Who Mourns for Adonais?"; TNG: "Captain's Holiday", "Power Play")
Later that year, after the USS Enterprise had been stopped by an unknown force, Captain Kirk ordered Lieutenant Nyota Uhura to relay the position and circumstances of the starship to Starbase 12. (TOS: "Who Mourns for Adonais?")
In 2364, the starbase was inexplicably evacuated for two days. Starfleet Command gave no explanation for this action. Captains Walker Keel, Rixx and Tryla Scott saw this as further evidence of a conspiracy within Starfleet. (TNG: "Conspiracy")
In 2369, Starbase 12 sent a message on Starfleet Commnet One via an encoded beta. This base was named on the chart Sector Ipai Relay Log - Relay Message Flow 1293 in Relay Station 47 Ops. (TNG: "Aquiel", production art)
Later that year, when Lieutenant Aquiel Uhnari and Klingon commander Morag were believed to be coalescent organisms, the USS Enterprise-D would have ferried these two to a secure medical facility on Starbase 12. (TNG: "Aquiel", okudagram)
Starbase 12 was also featured in the video game Star Trek: Bridge Commander and the novel Ship of the Line, in which the USS Bozeman protected Starbase 12, which was the building site of the USS Enterprise-E.