The TARDIS
The TARDIS is an acronym for the device called a Time And Relative Dimension In Space machine, or TT Capsule, used as a space-time vehicle by the Time Lords of Gallifrey. The Doctor's first companion, Susan Foreman, claimed to have coined the name TARDIS from the ships initials, however other Time Lords seemed perfectly familiar with the term as well as other species knowing it without being prompted. Occasionally the fourth word is pluralised to Dimensions, however it appears to be that Dimension, singular, is the more accurate as the Doctor's Fifth, Eighth, Ninth and Tenth incarnations have all referred to is singularly, and the plague visible during the Eleventh Doctor's time in the TARDIS reads "Time And Relative Dimension In Space". TARDISes travel through time and space by "disappearing there and reappearing here", a process called dematerialisation and rematerialisation. The process is controlled by the TARDISes dematerialisation circuit, though they are also capable of flight through space like conventional spacecraft, though doing so for prolonged periods of time can result in damage to the ship. During flight, many TARDISes owned by Time Lords make a distinctive grinding and whirring noise. Though River Song demonstrated that the TARDIS was capable of silent flight, teasing the Doctor by indicating that he left the brakes on causing the noise, other TARDISes owned by the Master and the Rani were known to make the sound in the course of normal operation and even the Minyans' computer could recognise the sound as being associated with the Time Lord ships. The Doctor is however, capable of working his TARDIS without the noise on occasion. The noise of a TARDIS is also made by other devices, such as the SIDRATs used by the War Lords, and a new dematerialisation circuit itself arrived using this noise when it was given to the third Doctor. On materialisation, TARDISes can often cause small tremors and strong winds in the areas in which they appear, though this can often go unnoticed. If a TARDIS materialises in the same part of space already occupied by another object, this object may well appear inside the TARDIS. The Eleventh Doctor also managed to successfully pilot an incomplete TARDIS into his own TARDIS and on top of an Ood, though in this instance the Ood was atominised. If a TARDIS materialises on top of another TARDIS, then a recursive loop can be formed whereby you enter that TARDIS inside your TARDIS, and inside there will be another TARDIS, and so on, as seen in Logopolis (Classic Series) when the Doctor materialised around the Master's TARDIS accidentally. However, if a TARDIS lands in the same space and time as another TARDIS, a Time Ram occurs, which can destroy both TARDISes along with its occupants and cause a black hold that could destroy the universe. TARDISes are capable of dematerialising and leaving the occupants behind, but have difficulty materialising in Time Loops.
TARDISes were used as a means for observation in various times and places, though were occasionally modified over the years. A modified TARDIS would receive a different type number to the previous TARDIS model. The Doctor owns a Type 40 TARDIS, which were decommissioned by the Doctor's Fourth incarnation. It may have been Time Lord policy to decommission old TARDIS types as a way of policing time travel, and may have kept a cap on the number of TT Capsules in use at any given moment in time. The policing of TARDISes was assisted by individual units of the same model could be opened by using the same key, so a key for one Type 40 TARDIS would open all Type 40 TARDISes. A key feature of all TARDISes was that the interior existed in a different dimension to the exterior, allowing them to be bigger on the inside than they were on the outside. The Fourth Doctor explained this to his companion by using two boxes of different sizes, and placing the bigger box down and standing back, until it looked like it could fit into the smaller box. Another key feature of TARDISes was their ability to blend into their surroundings by the use of the chameleon circuit. This would detect their surroundings before landing, and change the exterior to resemble something common to the landscape. It is suggested that the Time Lords in charge of the TARDIS can ultimately chose its exterior if the chameleon circuit is working properly.
TARDISes are said to be grown, rather than built, making them biological beings as opposed to artificial. It was once suggested that it was their complicated construction that gave an air of being biological, but it has since been revealed that TARDIS have a conscious mind and are capable of acting of their own accord. Such actions undertaken by TARDISes without prompting include resurrecting the Doctor's companion, Grace Holloway, and passenger, Chang Lee in the 1996 Television Movie (Classic Series). TARDISes would often mourn for the death of their Time Lord pilots, with some even committing suicide by flying into a sun, or hurling themselves into the Time Vortex. The conscious of the Doctor's TARDIS was placed inside a human female where we learned that all TARDISes are sentient beings. While inside the human female body, the Doctor's TARDIS remarked that "all of my sisters are dead", implying that TARDISes are female in The Doctor's Wife (Series 6 : Episode 4). Before a TARDIS becomes fully functional, it needed to be primed with the biological imprint from the symbiotic nuclei of the Time Lord's cells that would be using the TARDIS. This was known as the Rassilon Imprimatur and gave the Time Lord pilots a symbiotic link with their TARDIS, allowing them to survive the physical stresses of time travel. Without this Imprimatur, molecular disintegration would result, a safeguard itself against time travel, even if the TARDIS technology was copied. Once a TARDIs was properly primed and the imprint stored on the briode nebuliser, it could be safely used by any species.
During the Doctor's Second incarnation, a renegade Time Lord known as the War Chief provided time ships to the War Lords called SIDRATs to further their plans for conquest. When the Time Lords learned of this, they placed the War Lords planet in a time loop. When the Daleks also gained time travel, they made a machine similar to a TARIDS and in an alternate reality UNIT, under the command of the Doctor's former companion, Rose Tyler, used technology take from the Doctor's dying TARDIS to create a time machine to send an alternate Donna Noble back in time to correct her time line and history. Other specific TARDISes include the Master's and the Rani's. The Master is thought to have possessed at least two TARDISes, which had, according to the Doctor, a Mark II dematerialisation circuit. The Rani and the Monk both owned TARDISes as well.
The First Doctor obtained the TARDIS through theft from a museum, though it was revealed that the TARDIS wanted to be stolen, as she wanted to leave Gallifrey and so "stole" the only Time Lord mad enough to travel with her. When the TARDIS landed in London, 1963 with the First Doctor and Susan Foreman, it took the form of a police box to blend in with its surroundings. Due to an unknown reason, following this the chameleon circuit broke, resulting in the TARDIS exterior becoming stuck as a police box. The Doctor has tried on two occasions to fix the circuit, the first in Logopolis, which resulted in his death and regeneration from his Fourth incarnation to his Fifth, and the second was during his Sixth incarnation. During this adventure, despite fixing the chameleon circuit to allow the exterior to change, the Doctor had no apparent control over what it changed into, suggesting that not all of it was fixed. It promptly broke again soon after. Any TARDIS requires 6 pilots to fly successfully, though it can be flown with less and often, the Doctor flies it alone, resulting in some poor piloting on occasions. However, it was also revealed by the TARDIS when it was trapped in a human's body, that it doesn't always take him where the Doctor wants to go, but to where he needs to go, to save people or other such events. The TARDIS stated that she had known the Doctor for about 700 years, indicating that the Doctor was about 200 years old when he first acquired her. The TARDIS also notes that the Doctor has always pushed her doors open, despite the instructions on the door saying "Pull To Open" and said that it irritates her, as well as the Doctor's defiance for instructions in general. The Doctor once admitted he had thrown the TARDIS' instruction manual in a supernova because he disagreed with it. Following the Last Great Time War, the Doctor believes that his Type 40 TARDIS is the last in existence. However, the revelation that the Master survived the Last Great Time War might suggest that his TARDIS is around somewhere, though he abandoned the end of the universe in the Doctors TARDIS, thus leaving his behind. If he escaped the Last Great Time War in a TARDIS, logic would dictate that his TARDIS is available somewhere, though without its Time Lord pilot, it may be dying or dead.