Theta Sigma
Theta Sigma, informally shortened to Thete, was the name by which the Doctor's old Academy schoolmate Drax knew him. The Fourth Doctor repeatedly corrected him when they met again centuries later, wanting to be called Doctor instead. (TV: The Armageddon Factor [+]Loading...["The Armageddon Factor (TV story)"])
Some accounts treated "Theta Sigma" as the Doctor's actual, proper name as far as Gallifrey was concerned; for example, a communication from Gallifrey Databank to Gallifrey High Command noted that K9 Mark I had been modified by "Time Lord Theta Sigma (also known as 'the Doctor')". (PROSE: K9 and the Time Trap [+]Loading...{"page":"1","1":"K9 and the Time Trap (novel)"}, K9 and the Beasts of Vega [+]Loading...{"page":"1","1":"K9 and the Beasts of Vega (novel)"}, K9 and the Zeta Rescue [+]Loading...{"page":"1","1":"K9 and the Zeta Rescue (novel)"}, K9 and the Missing Planet [+]Loading...{"page":"1","1":"K9 and the Missing Planet (novel)"}) In an alternate timeline in which Rassilon failed to finish the Eye of Harmony before his death, the Doctor never left Gallifrey and became a commentator rather than a renegade Time Lord. He was known as Commentator Theta Sigma. (AUDIO: Forever [+]Loading...["Forever (audio story)"])
"ΘΣ" was part of River Song's message to the Eleventh Doctor on the universe's oldest cliff-face on Planet One. (TV: The Pandorica Opens [+]Loading...["The Pandorica Opens (TV story)"]) River Song claimed and demonstrated on numerous occasions that she knew the Doctor's secret real name. (TV: Forest of the Dead [+]Loading...["Forest of the Dead (TV story)"], The Name of the Doctor [+]Loading...["The Name of the Doctor (TV story)"], etc.)
However, the Seventh Doctor would later claim that "Theta Sigma" was his "nickname at college" to the coincidentally-named Trevor Sigma. (TV: The Happiness Patrol [+]Loading...["The Happiness Patrol (TV story)"]) The Ninth Doctor would later reiterate this in a duel with the Grimminy-Grew to guess the other's true name, saying that "old school nicknames don't count". (AUDIO: Station to Station [+]Loading...["Station to Station (audio story)"])
In one account, the Doctor considered it a "Time Lord coding", which he couldn't "bear [to be] addressed as". (PROSE: Doctor Who and the Armageddon Factor [+]Loading...["Doctor Who and the Armageddon Factor (novelisation)"]) It identified him uniquely amongst the Time Lords and was not to be spoken outside of the Academy. (PROSE: Falls the Shadow [+]Loading...["Falls the Shadow (novel)"]) According to the author of The Time Lord Letters, a historical document compiling writings by and concerning the Doctor, "Theta Sigma" was also their Academy Student Identification Code. (PROSE The Time Lord Letters [+]Loading...["The Time Lord Letters (novel)"])
The Eleventh Doctor briefly referred to himself as "Theta Sigma", as well as "the Other", during his intervention into the creation of the Type 1 TARDIS. Rassilon noted that he was acting just like the "chrono-scrollwriters of old" by refusing to give his real name. During his prolonged, time lock-defying stay on ancient Gallifrey, the Eleventh Doctor also advised Rassilon in technological matters, with Rassilon admiring the man's unorthodox genius but remaining wary of him, noticing a familiarity within the Doctor. (COMIC: The Lost Dimension [+]Loading...["The Lost Dimension (comic story)"])
In July 2016, the Twelfth Doctor joined an internet forum under the name ThetaSigma. He wrote an extremely negative post about an episode of a television series. (COMIC: Planet of the Rude [+]Loading...["Planet of the Rude (comic story)"])
When the Doctor's final incarnation permanently died during the first battle of the War in Heaven, his coffin had two Greek letters on it; one of these was "Sigma". (PROSE: Alien Bodies [+]Loading...["Alien Bodies (novel)"])
In one universe, the Eighth Doctor's counterpart was known as Theta Stigma. (COMIC: The Glorious Dead (part 5) [+]Loading...{"part":"5","1":"The Glorious Dead (comic story)"})
