Aliases of the Doctor

Throughout his travels in time and space, the Doctor took and was called by a number of different aliases, titles and names. Some were fleeting. Others, for example John Smith, were used by almost all of his incarnations. The Doctor told few individuals his real name, causing others to call him the Doctor.

Ubiquity of the title
"The Doctor" was not a name, but a title. His true name was not generally known, and "the Doctor" became an alias. It has been implied his given name was ceremoniously withdrawn and stricken by his Cousins as punishment for a disgrace he visited upon his House. (NA: Lungbarrow)

The Doctor's real name has been said to be difficult to pronounce, for humans at least. He once told an interrogator (when asked his name): "you wouldn't be able to pronounce the first syllable of it." (PDA: Salvation)

Even when he was twice put on trial by his own people, the Time Lords, he was only referred to as "the Doctor" (DW: The War Games), although the Valeyard, who prosecuted the second trial (and who, himself, was a future manifestation of the Doctor) acknowledged that it was an alias. (DW: The Trial of a Time Lord) However, as the Doctor chose that "name" as a Gallifreyan custom (DW: The Sound of Drums), its use in an official capacity is not exceptional. The Doctor kept his true name hidden despite numerous "mind-probe" attempts, voluntary (DW: The Girl in the Fireplace) and involuntary. (DW: The Shakespeare Code)

Individuals with knowledge of his name
When the Tenth Doctor first encountered her, River Song claimed to have known him at some point in his future. She was one of the few individuals confirmed to know his true name. To win the Doctor's trust, and prove her "credentials", she whispered his name in his ear. The Doctor seemed shocked at this, later saying to her "there is only one reason I would ever tell anyone my name, only one time I could." (DW: Forest of the Dead)
 * She whispered the Doctor's name for a duration of roughly two seconds, suggesting the name may only be a few syllables long (assuming she uttered a complete name). Steven Moffat made a point of mentioning that he always felt the Doctor's name was never revealed only because it must have been some "terrible secret".
 * She whispered the Doctor's name for a duration of roughly two seconds, suggesting the name may only be a few syllables long (assuming she uttered a complete name). Steven Moffat made a point of mentioning that he always felt the Doctor's name was never revealed only because it must have been some "terrible secret".

When asked about the Sixth Doctor's name, his companion Peri Brown once said that the Doctor had told her she would find it unpronounceable. (BBCR: Slipback)

The Eighth Doctor's companion Samantha Jones was told his real name. She found it quite alien and virtually unpronounceable. (EDA: Unnatural History, Vanderdeken's Children)
 * When the Doctor spoke his real name aloud in the novel Vanderdeken's Children it was not written in the prose, but represented by "—" instead.

Near the end of his tenth incarnation, members of an unidentified pan-dimensional race came to know the Doctor's real name. At this point he was of the opinion there was no one left in the universe who knew it. (NSA: The Last Voyage)

True name
The Carrionite Lilith, unable to discover the Tenth Doctor's true name even with the "witchcraft" used by her kind, remarked, "There is no name. Why would a man hide his title in such despair?" (DW: The Shakespeare Code) The psychically-gifted Evelina, who attempted to foretell the Doctor's future, remarked that his "true name" was "hidden" from her. (DW: The Fires of Pompeii)

The Eleventh Doctor once joked that his middle name was 'Bad Penny', and that he received strange looks when filling in forms. (DW: The God Complex)

According to Dorium Maldovar after his beheading, the religious organisation the Silence had a particular interest in the Doctor's name. He explained that if the Doctor lives long enough, on the fields of Trenzalore, at the fall of the Eleventh, when no creature could speak falsely or fail to answer, a question that must never be answered would be asked: the first question, hidden in plain sight. The question is "Doctor Who?", implying that, for reasons unknown, the Silence wanted to prevent the Doctor from revealing his true name. (DW: The Wedding of River Song)

Significance
River Song said that the Doctor was the first to have this title. The rest of the universe later adopted it, usually to mean 'healer' or 'wise man'. In some parts of the universe (eg. the Gamma Forest), it came to mean 'mighty warrior'. (DW: A Good Man Goes to War) The Master implied that the Doctor's title had been chosen because it meant "the man who makes people better". (DW: The Sound of Drums)

Far from being a randomly chosen title, or one intended to inflate his credentials, the Doctor did possess at least one doctorate. (DW: The Moonbase, The Armageddon Factor, The Mysterious Planet) He sometimes described himself as a "Doctor of many things" (DW: Four to Doomsday) or "everything". (DW: Utopia) Exactly what his doctorate(s) were for remain a mystery, but on several occasions the Doctor stated he was not a medical doctor. That didn't stop his second, third, tenth and eleventh incarnations from sporting a stethoscope on occasion. The Doctor did note that he had studied medicine in the 19th century. (DW: The Moonbase) He also claimed one of his doctorates to be in in cheese making. (DW: The God Complex)

List of aliases
During his life, numerous names, titles and epiphets were either used by or bestowed upon the Time Lord most widely known as the Doctor.

The Doctor
As noted above, The Doctor itself was an alias, despite it being the title by which the Time Lord was most widely known. It has been claimed that, as a Gallifreyan custom, he chose this alias himself (DW: The Sound of Drums). It may have been a misunderstanding caused by an earlier alias. (See Names given by others.) On one occasion, a human knew this name without the Doctor saying it out loud. (NA: Cat's Cradle: Warhead) His other aliases often included the title "Doctor". (DW: Spearhead from Space) He implied to Peri his given name also began with such a title. (DW: The Mysterious Planet) Drax, a fellow Time Lord, indicated the Doctor did indeed possess a doctorate (although in what was not indicated). (DW: The Armageddon Factor) According to one account, during his first incarnation, the Doctor adopted this name in dealing with human colonists on the planet Iwa at the same time that his granddaughter adopted the name "Susan". (TN: Frayed)

During his eleventh incarnation, the Doctor claimed that he didn't actually know why he and other people called him 'The Doctor'. (DW: The Lodger)

John Smith
John Smith was an alias the Doctor would frequently use on Earth and around humans when a "standard" name was needed. It was often preceded by the title "Doctor", though not always - for example, when he was undercover as a teacher at a school or a patient in a hospital. (DW: School Reunion, Smith and Jones) The earliest known use occurred during his first incarnation, when he used a library card with the name Dr. J. Smith while living at 76 Totter's Lane (DW: The Vampires of Venice), as well as for identification when renting the junkyard. (ST: The Rag and Bone Man's Story) In his second incarnation, the name was independently used by his companion Jamie McCrimmon (who would not have known of the Doctor's earlier use of the name). (DW: The Wheel in Space) Later, he adopted it on a semi-regular basis during his third incarnation while exiled on Earth, when he served as unpaid scientific advisor to UNIT. (DW: Spearhead from Space, et al)

As "John Smith" was considered a generic name in some Earth cultures, the Doctor's use of the alias was occasionally treated with scepticism. (DW: Midnight)

The Doctor twice changed himself into a human who used the name John Smith. This occurred in his seventh incarnation (NA: Human Nature) and in his tenth (DW: Human Nature/The Family of Blood).

Known uses of John Smith

 * On the First Doctor's library card while living at 76 Totter's Lane (DW: The Vampires of Venice)
 * As identification when renting the junkyard from its owner, Hawkins (ST: The Rag and Bone Man's Story)
 * When Rebecca Nurse asked his name, realising that a simple 'Doctor' would not do (PDA: The Witch Hunters)
 * To check in at Ingersoll's tavern in Salem (PDA: The Witch Hunters).
 * Given to him by Jamie and used aboard Space Station W3. (DW: The Wheel in Space)
 * When questioned by a German soldier, who replies "Good. Now we are getting somewhere". (DW: The War Games)
 * During his exile on Earth with UNIT, when he served as its scientific advisor (DW: Spearhead from Space, The Time Warrior)
 * On the planet Kastopheria when asked by Admiminstrator Charteris, who reponded with "A large family, the Smiths" (PDA: Catastrophea)
 * "JSMITH" was the Sixth Doctor's username online. (PDA: Blue Box)
 * A German variation of this name, "Johann Schmidt," was used by the Sixth Doctor when he infiltrated the Third Reich to investigate the origins of the Fourth Reich that was currently active in 2001, creating fake credentials establishing him as Major-General Johann Schmidt of the Berlin Fifth Medical Corps (PDA: The Shadow in the Glass). The Seventh Doctor also used this name while impersonating a Nazi Reichsinspektor (NA: Timewyrm: Exodus), while the Eighth Doctor used it while posing as a German spy. (BFA: Storm Warning) An alternative version of the Eighth Doctor who existed in a timeline where the Nazis won World War II, also used the name. (BFA: Colditz, Klein's Story) The Eleventh Doctor took the place of a real Dr. Schmidt when his psychic paper displayed the credentials of a visitor expected at the Lexington Bank. (NSA: Borrowed Time)
 * A French variation, "Jean Forgeron," was used by the Fourth Doctor when met by French soldiers in the labyrinth underneath Paris. (IDW: The Forgotten)
 * Given to him coincidentally and serendipitously by Chang Lee on his hospital admittance forms after he was shot near the end of his seventh incarnation. (DW: Doctor Who)
 * Appeared as "Dr. John Smith" on a piece of psychic paper during a visit to 1941 (although he never actually used the alias on this occasion). (DW: The Empty Child)
 * While working undercover as a science teacher at Deffry Vale High School. (DW: School Reunion)
 * While posing as a patient at Royal Hope Hospital. (DW: Smith and Jones)
 * On both the Confederate and Union sides of the American Civil War as a guise under which the Fifth Doctor could render medical assistance, while having the freedom to search for his lost companions, Peri and Erimem. He received some local publicity under the name after delivering the baby of the mayor of Gable, West Virginia. He was personally known to Abraham Lincoln by this name after he had saved the president's life during an assassination attempt in Richmond, Virginia. (TN: Blood and Hope)
 * While impersonating a health and safety officer during an investigation of Adipose Industries. (DW: Partners in Crime)
 * The Tenth Doctor attempted to use the alias aboard the Crusader 50 bus on the planet Midnight, but it was recognised as a false name and rejected by the panicking humans aboard. (DW: Midnight)
 * When "reintroducing" himself to Donna Noble after he erased her memories of him. (DW: Journey's End)
 * When encountering Jackson Lake, a man who, due to Infostamp exposure, believed himself to be an incarnation of the Doctor. (DW: The Next Doctor)
 * While posing as his Ganger counterpart as an independent name for himself. (DW: The Almost People)

Theta Sigma
Theta Sigma (ΘΣ), informally Thete, was a nickname of the Doctor at the Time Lord Academy on Gallifrey. (DW: The Armageddon Factor, The Happiness Patrol, DWMS: Flashback) It might also have been a codename used by some branch of Time Lord government to refer to the Doctor. (The Adventures of K9 series)

River Song used the written form "ΘΣ" in a message to the Eleventh Doctor on Planet One, though these and other apparently Greek numbers on the cliff face may have been the "co-ordinates" there the Doctor later referenced. (DW: The Pandorica Opens)

In an alternative 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. (BFG: Forever)


 * "ΘΣ" was also a commonly-used abbreviation for "God" in Greek manuscripts of the New Testament. The capital letters Theta and Sigma together resemble the word "ΟΣ", (who), and some scribal variations in New Testament texts will have "ΟΣ" in a few passages while others have "ΘΣ" in the same passages.

Minor aliases
A list of aliases the Doctor has used in his travels.

First Doctor

 * Maximus Pettulian: While visiting the Roman Empire before the Great Fire of Rome, the Doctor briefly impersonated a murdered lyre player named Maximus Pettulian in the court of Nero until his true identity was uncovered. (DW: The Romans)
 * Zeus: When the Greek warrior Achilles mistook the Doctor for Zeus posing as an old man, he went along with it, until the unconvinced Agamemnon spoiled the Doctor's ruse. (DW: The Myth Makers)
 * Doctor Caligari: used by the First Doctor when he arrived in Tombstone, Arizona, impersonating a magician. It did not take; the locals mistook him for Doc Holliday. (DW: The Gunfighters)

Second Doctor

 * Doctor von Wer: The German translation of 'Doctor [of] Who.' The Second Doctor briefly used this during his visit to Scotland in 1746. He also signed a note Doctor W around this time. (DW: The Highlanders)

Fourth Doctor

 * ?: The Doctor signed a document with a question mark prior to the Sontaran invasion of Gallifrey. (DW: The Invasion of Time) Later, the Seventh Doctor signed a document the same way during a visit to London in 1963. (DW: Remembrance of the Daleks)

Fifth Doctor

 * Doctor Walters: The Doctor used this name while stuck in Victorian London without his TARDIS for a year from November 1866 - 1867. (BFA: The Haunting of Thomas Brewster)


 * The Supremo: The Doctor called himself "the Supremo" while leading the alliance against the army of the renegade Time Lord Morbius. Originally, his title was "Supreme Controller", but the Ogrons of his personal guard could not pronounce it and shortened it to the simpler "Supremo". (PDA: Warmonger)

Sixth Doctor

 * Doktor of TARDIS: Used by the Sixth Doctor to move discreetly around an alternate version of the city of Rome. (MA: State of Change)
 * The Sandman: Adopted as part of a plan to stop an alien race attacking others by inspiring the idea of him as a monster. (BFA: The Sandman)
 * Claudius Dark: Used while the Doctor was undercover in the 19th century, so as to avoid the attention of Kempston and Hardwick. (JAL: The Hourglass Killers)

Seventh Doctor

 * Muldwych: This was, the Seventh Doctor believed, an alias used by a future and/or alternative Doctor in a magic-using parallel Earth. (DW: Battlefield) This was, apparently, a future incarnation of the Doctor who, at times, also used the name. (NA: Happy Endings)
 * Professor: This was what Ace called The Doctor in Dragonfire though to Survival.

Eighth Doctor

 * Doctor Bowman: Used to get close to the atomic clock in San Francisco. (DW: Doctor Who)
 * Gracie Witherspoon: To defeat the Threshold, the Doctor used a personal chameleon circuit to take the appearance of a female Threshold agent while Shayde masqueraded as the Doctor. (DWM: Wormwood)
 * Dr. Friedlander: While visiting Banquo Manor (There was a real Doctor Friedlander who hadn't arrived at the Manor; the Doctor simply assumed the name to avoid having to explain his presence). (EDA: The Banquo Legacy)
 * The Bringer of Darkness, the Oncoming Storm and the Evergreen Man: all epithets he offered Slake. He also gave his full, former Time Lord title as President of the High Council of the Time Lords, Keeper of the Legacy of Rassilon, Defender of the Laws of Time and Protector of Gallifrey. (EDA: Vampire Science)
 * Perdix: while in Rhadamanthys' court. (TN: Fallen Gods)
 * James Alistair Bowman: while on Ha'olam. (EDA: Seeing I)
 * Johnsmith8: When hunting IamI in a chat room. (ST: Lonely)
 * Sir Doctor Peter Pollard: when he arrived at Sebastian Grayle's 19th centuary manor house, with Charllote Pollard pretending to be his daughter. (BFA:Seasons of Fear)

Ninth Doctor

 * Mr. Spock: Rose introduced the Doctor as this to Capt. Jack Harkness when she was still pretending the two of them were time agents. (DW: The Empty Child)
 * Hal Gryden: While on the planet Arkannis Major, the Doctor adopted the name of this fictional newscaster to broadcast a message of peace to the people of the world. (NSA: The Stealers of Dreams)
 * The Oncoming Storm: an epithet the Doctor said the "legends of Skaro" call him whilst taunting the Daleks. (DW: The Parting of the Ways)

Tenth Doctor

 * Doctor James McCrimmon: The Tenth Doctor briefly called himself Dr. James McCrimmon while visiting Scotland in 1879; this was the name of one of his old companions. Later, he dropped the alias and was knighted by Queen Victoria as "Sir Doctor of TARDIS". (DW: Tooth and Claw)
 * The Oncoming Storm: Rose Tyler called the Doctor by this name when she and Mickey Smith were captured by the clockwork droids and being prepared for dissection and was threatening them using that title. (DW: The Girl in the Fireplace ) When he was interrogating a Dalek aboard of the Wayfarer the Doctor used this title to introduce himself to the Dalek (NSA: Prisoner of the Daleks)
 * Sir Doctor of TARDIS: After being knighted under this name, the Doctor tried to use it when he met William Shakespeare. Shakespeare's mind could not be swayed by psychic paper. (DW: The Shakespeare Code)
 * Doctor Vile: The Doctor briefly masqueraded as the space pirate Doctor Vile to stop a war. (DW: The Infinite Quest)
 * Doctor McCoy: After a discussion with Donna Noble, in which she compared the sonic screwdriver to Star Trek's tricorder, the Doctor used the alias Dr. McCoy (after Dr. Leonard "Bones" McCoy) when dealing with a group of soldiers. (Donna called herself Captain Kirk, after James T. Kirk). (NSA: Pest Control)
 * Spartacus: When visiting Pompeii in 79 A.D, the Doctor and Donna both called themselves 'Spartacus'. (DW: The Fires of Pompeii)
 * Chief Inspector Smith from Scotland Yard: Used when involved with the death of Professor Peach. (DW: The Unicorn and the Wasp)
 * Doctor Noble The Doctor used this when investigating the Ood Industries claiming Donna and he were from the Noble Corporation(DW: Planet of the Ood)
 * Actors: When visiting Hollywood in the early 20th century, the Doctor adopted a number of Hollywood-related names whilst attending a party. Within the space of a few minutes, he called himself "Tom Cruise," "Michael Caine," "Pee-Wee Herman," and "Tom Hanks." (IDW: Silver Scream)

Eleventh Doctor

 * The Oncoming Storm: The Doctor called himself this when he misinterpreted one of Craig's football mates asking for help in 'annihilating' another team. (DW: The Lodger)
 * Captain Troy Handsome of International Rescue: When introducing himself to a crashed time ship's avatar. (DW: The Lodger)
 * Commander Bond of Naval Intelligence: An alias used by the Doctor when he encountered Gein. (DWAN: Sub-species)
 * Caretaker: The alias used when Madge, Lily and Cyril Arwell stayed with him in a dilapidated house in Dorset. (DW: The Doctor, the Widow and the Wardrobe) In the same episode, he also follows this up by saying he is either called the Caretaker or "Get off this planet." "Though," as he points out, "strictly speaking that probably isn't a name."

Names given by others
A list of names other individuals have called the Doctor throughout his travels.
 * The Doctor: While some accounts contradict this, Barbara Wright and Ian Chesterton may have accidentally given him this title, after being informed that his alias of "Doctor Foreman" was not his real name and given no other name to call him (DW: An Unearthly Child). The Doctor did not protest and, by the end of the sixties, starts introducing himself by this name.
 * Doctor Who: The name Doctor Who was applied to the Doctor, intentionally or unintentionally, on many occasions. The computer WOTAN once referred to the Doctor as Doctor Who, for reasons which have never been explained. (DW: The War Machines) Clive Finch also used this name when referring to the Ninth Doctor on his website Who is Doctor Who?. (DW: Rose) K9 was heard on occasion making playful remarks related to the Who name. (KAC: A Girl's Best Friend, SJA: Invasion of the Bane) It is unclear whether the Doctor himself ever actually used the name, although his third incarnation drove an automobile with the licence number "WHO 1" (DW: Doctor Who and the Silurians, et al) and not long before his regeneration was seen driving a futuristic vehicle that possibly apocryphal sources suggest may have been called the Whomobile. (DW: Invasion of the Dinosaurs) The Doctor wrote a series of children's books during his time with UNIT which were mistakenly published under this name. (BFA: The Kingmaker)


 * The name was often unintentionally invoked when characters wished to learn who the Doctor was and what his name was: "Doctor? Doctor who?" Often this is done for comic effect; see The "Doctor Who?" running joke.

First Doctor

 * Doctor Foreman: Used by Ian Chesterton and Barbara Wright, who assumed he shared Susan's surname. (DW: An Unearthly Child).
 * The Traveller from Beyond Time: The epiphet given to the Doctor by the Elders. (DW: The Savages)

Second Doctor

 * Ka Faraq Gatri (Destroyer of Worlds): As early as his second incarnation, the Doctor knew that the Daleks had given him this epiphet, and the Daleks used it prior to the destruction of Skaro. (DWM: Bringer of Darkness)
 * Doctor Galloway: Edward Waterfield made up this name for the Second Doctor when arranging to meet him. (DW: The Evil of the Daleks)

Third Doctor

 * Quiquaequod: Miss Hawthorne referred to the Doctor as "the great wizard Quiquaequod" while attempting to bluff the villagers of Devil's End into releasing him. (DW: The Dæmons)

Fourth Doctor

 * The Evil One: The Sevateem tribe referred to the Doctor as The Evil One. The supercomputer they worshipped had two personalities, one of which was based on the Fourth Doctor. (DW: The Face of Evil)
 * The Watcher: To refer to a projection of the Doctor partway between his fourth and fifth incarnations, before they realised his true identity. The only individual heard to utter it was Adric. (DW: Logopolis)

Sixth Doctor

 * Old One: Given by Balazar while investigating Ravalox. (DW: The Mysterious Planet)
 * The Sandman: The Sixth Doctor's actions in the past of the Galyari led to his entering their race memory as a legendary monster called the Sandman. (BFA: The Sandman)
 * The Dark One / He Whose Name Dare Not Be Mentioned: Names given to the Sixth Doctor/the Valeyard in the legends of the Great Kingdom, an unstable alternate version of London created by the botched summoning of Saraquazel. (MA: Millennial Rites)

Seventh Doctor

 * The Oncoming Storm: was also the epithet given to him by the Draconians, ("Karshtakavaar" in Draconian) who were generally the Doctor's allies, to indicate the traumatic events which so often accompanied his arrival. (NA: Love and War)
 * Time's Champion: A title given to the Doctor for his service to Time during his seventh incarnation. (NA: Love and War)
 * Ka Faraq Gatri (other Dalek names): The Daleks referred to the Doctor as the Ka Faraq Gatri, which may mean Bringer of Darkness and/or Destroyer of Worlds. By implication, the epiphet Destroyer of Worlds referred to the Doctor's destruction of the Dalek home planet Skaro in his seventh incarnation, an act for which he would feel some remorse. (DW: Remembrance of the Daleks, NA: Timewyrm: Revelation)
 * Snail or Wormhole: Given to him by his cousins of the House of Lungbarrow, because, unlike Loomed Gallifreyans, he had a bellybutton. (NA: Lungbarrow)
 * Merlin: The Doctor was mistaken by the people from an alternate universe which inspired the Arthurian legend to be Merlin. It is intimated that a future or alternate version of the Doctor may in fact be this Merlin. (DW: Battlefield)
 * John Doe: The customary name in America for an unidentified human male was applied to the apparently dead Doctor by the staff of Walker General Hospital, and he was given a "John Doe" toe tag before being wheeled into the morgue's freezer. (DW: Doctor Who)

Eighth Doctor

 * Doctor Bowman/James Alistair Bowman: Dr. Grace Holloway came up with this name for the Eighth Doctor whilst introducing him to others. (DW: Doctor Who) The Doctor subsequently used this alias on occasion, filling it out to James Alistair Bowman. (EDA: Seeing I)

Ninth Doctor

 * Mr Spock: Rose Tyler, annoyed with the Ninth Doctor not telling her his name, introduced him to Captain Jack as Mr. Spock, after the character of the same name from Star Trek. (DW: The Empty Child)
 * The Oncoming Storm: The Daleks also referred to the Doctor as the Oncoming Storm in the myths of their homeworld, as noted by his ninth incarnation, indicating that the Daleks feared him. (DW: The Parting of the Ways)

Tenth Doctor

 * Merlin: The Tenth Doctor would later claim that he was called Merlin by King Arthur. (IDW: Fugitive)
 * The Oncoming Storm: Used by Rose when trying to threaten the Clockwork Droids in freeing her and Mickey, only to fail when the Doctor arrives, apparently drunk, as a distraction. (DW: The Girl in the Fireplace)
 * Pretty Boy: in (DW: Silence in the Library) Dr.Riversong kept referring to him as such in the beginning of the episode.  "Ohh i'm pretty boy."

Eleventh Doctor

 * The Raggedy Doctor: (also, Raggedy Man) A nickname given to the Eleventh Doctor by Amy Pond. Amy told other people in Leadworth and they referred to him in this way when they met him. (DW: The Eleventh Hour) Amy also called him "Raggedy Man" in The Big Bang and The Girl Who Waited.
 * Caesar: A Roman Auton, under the influence of River Song's Hallucinogenic lipstick, in 102 A.D., mistook the Doctor for Caesar. (DW: The Pandorica Opens)

Behind the scenes
The first edition of the behind-the-scenes book The Making of Doctor Who, published in 1972, stated that the Doctor's name was "δ³Σx²". This has never been confirmed in any Doctor Who narrative, but these letters do appear on the plinth in the Tomb of Rassilon in DW: The Five Doctors. They are also seen on K9's regeneration unit in K9TV: Regeneration.