TARDIS key

A TARDIS key was the key to the exterior door of a TARDIS. They could be found in a variety of shapes and designs. Whatever the key's shape and size, and whatever form the TARDIS' outer plasmic shell, the key would fit a lock somewhere in the structure.

Function
Use of a TARDIS key enabled entry to a locked TARDIS in the manner of other types of keys, and was not limited by the reconfigurable shape of either the key itself or the outer plasmic shell of the TARDIS. Different TARDIS shells had different locks, whether obvious or hidden, and a TARDIS lock would accept its key even if it was physically larger than the lock. (PROSE: Interference - Book Two)

The barrier on the Doctor's TARDIS was a double curtain trimonic. As such, it utilised a cypher indent key. (TV: The Deadly Assassin). In its police box disguise, entry was allowed by inserting a key into the lock, just like a real police box. (TV: Doctor Who, Rose and others) However, the locking mechanism did not respond to police-issued keys, (TV: Blink) and would not even open when unauthorised persons used the Doctor's keys. According to Susan Foreman, the lock had a defence mechanism in which there were twenty one different holes inside the lock, one being the right place and other twenty being wrong. If a mistake was made, the whole inside of the lock would melt. (TV: "The Survivors") The Brigadier was prevented from opening the TARDIS in the Third Doctor's absence, because of the metabolism detector on the lock. (TV: Spearhead from Space) Nevertheless, there were occasions on which strangers were able to successfully use the Doctor's key, suggesting either a flaw in the metabolism detector or a choice on the part of the TARDIS. (TV: The War Machines, Doctor Who, Blink, HOMEVID: Shada) The lock could be manually secured from inside the TARDIS, preventing even authorised individuals from using the key to unlock the doors from the exterior. (TV: The Dalek Invasion of Earth, Utopia) At some point shortly before the Tenth Doctor's regeneration, the TARDIS had a system that allowed him to lock the TARDIS remotely using a fob (as a joke, the TARDIS roof light flashed and a alarm chirp sound was heard, similar to that used on vehicles on Earth). He was also able to open the door remotely. (TV: The End of Time)

There were occasions on which the Doctor or his companions needed to use the key to lock the doors, (TV: The Sensorites) while on others (TV: Spearhead from Space, The Christmas Invasion) the act of merely closing the doors locked the TARDIS. It was evident that the TARDIS' lock could be set to secure either automatically or manually.

However, a key was not always needed to open a TARDIS. During the Doctor's first encounter with River Song (from his point of view), she remarked that she had witnessed his future self open the doors of the TARDIS by snapping his fingers. The Doctor reacted with disbelief, but later successfully opened the doors in this fashion. (TV: Forest of the Dead) After regenerating into his eleventh self, the Eleventh Doctor again opened the TARDIS by snapping his fingers, demonstrating that this method had become a reliable way for him to do so. (TV: The Eleventh Hour) The Doctor did, however, continue to make regular use of his key to unlock the TARDIS. (TV: The Time of Angels, Flesh and Stone, et. al.)

Design
The key to the Doctor's TARDIS took several forms.

The First and Second Doctors  used a key which resembled a plain Earth  lock, such as might be used with a genuine police box. The Third Doctor started out with the same key. (TV: Spearhead from Space)

At some point in his third life, the Doctor began experimenting with new designs for his key, coming up with several alternative designs before settling on a spade-shaped silver key. After his regeneration, the Fourth Doctor experimented with even more radical designs, which included a double-helix apparently made of plastic, before eventually going back to the spade and Yale styles. (PROSE: Interference - Book Two) The Yale key was also used by the Fifth and Sixth Doctors.

The Seventh Doctor used yet another design, incorporating the Seal of Rassilon with a fan-shaped motif. (TV: Ghost Light) The Seventh Doctor eventually went back to the spade key, which the Eighth Doctor initially continued to use. (TV: Doctor Who) At some point, the Eighth Doctor returned to using the Yale key. (PROSE: The Crooked World, History 101)

The Ninth, (TV: Aliens of London, Father's Day) Tenth, (TV: 42) and Eleventh Doctors (TV: The Eleventh Hour) used Yale-type keys in a gold/brass colour as opposed to earlier Doctors' silver-coloured Yale keys.

Abilities
Beyond simply unlocking the TARDIS doors, the Doctor's TARDIS key has often displayed other uses and characteristics.
 * The Doctor's TARDIS key could be used to open TARDISes other than his own. (TV: The Mark of the Rani)
 * The TARDIS key glowed and grew warm when the TARDIS was about to materialise nearby (TV: Aliens of London) or when an incapacitated TARDIS had finished rebuilding itself. (TV: The Eleventh Hour)
 * This feature was also exhibited when Susan Foreman's key, long disused, began to glow when the Eighth Doctor came back to Earth during the Daleks' second attempt to move the planet in the 22nd century. (AUDIO: Lucie Miller)
 * When the TARDIS fell through a wound in time, the Doctor realised he could use the key - still warm and in contact with the ship - to bring it back. (TV: Father's Day)
 * If modified properly, the TARDIS keys exhibited perception filter properties similar to that of the TARDIS. (TV: The Sound of Drums)
 * The key could remotely lock the TARDIS from a distance, similar to a car alarm. (TV: The End of Time)
 * The key could be used to place the TARDIS one second out of sync with the rest of the universe, preventing it from being seen. (TV: The End of Time)
 * The key could be modified to track and locate the TARDIS, allowing the Doctor to find the TARDIS if it was within a hundred years of his position. (COMIC: The Forgotten)
 * When Sarah Jane Smith left the key in the lock without opening the door, it cancelled the pause control. The TARDIS travelled to its set coordinates on Earth, instead of remaining in the copy of Devesham created by the Kraal. (TV: The Android Invasion)
 * The Eleventh Doctor was able to summon the TARDIS to him using the key. (TV: The Time of the Doctor)
 * The keys were very resilient and could only be destroyed by being dropped into lava. (TV: Dark Water)

Known copies
The Doctor usually had his TARDIS key on his person at any given time. There were numerous other known copies of his key including, but probably not limited to:
 * A spare key made by the First Doctor to replace one Marco Polo had taken away. Marco took away the spare as well and later returned both. (TV: Marco Polo)
 * Susan's first key was destroyed by the Doctor whilst shorting out static electricity production in the Dalek City. Just before he destroyed it, he promised to make her a new one. (TV: The Daleks)
 * Susan's second key was seen when Susan locked the TARDIS aboard a human spaceship in proximity of the Sense Sphere. (TV: The Sensorites) Later, knowing she would likely never see the TARDIS again, she intentionally abandoned this key in the dirt on the bank of the River Thames, after her grandfather helped her decide to start a new life in the 2160s by locking her out of the TARDIS. (TV: The Dalek Invasion of Earth) Susan later recovered her key. (AUDIO: Lucie Miller)
 * A key that fell from the Doctor's cloak and was picked up by Ben Jackson. (TV: The War Machines)
 * During their travels with the Fifth Doctor, both Tegan Jovanka and Nyssa were given their own keys to the TARDIS (AUDIO: Hexagora) as was Peri Brown. (PROSE: Fascination) Peri still had her key when travelling with the Sixth Doctor. (PROSE: House)
 * A spare copy of the spade-type key kept by the Seventh and Eighth Doctors was stored in a secret compartment in the TARDIS' outer shell. It was used by the Eighth Doctor when Chang Lee had stolen the original from his possessions. (TV: Doctor Who)
 * A double-helix shaped key, apparently made of plastic, which Sarah Jane Smith took with her when she left the TARDIS. (PROSE: Interference - Book Two)
 * Anji Kapoor's Yale design key. (PROSE: The Crooked World)
 * A spade key worn on a choker by President Romana (WC: Shada)
 * The Yale-type keys given to Rose Tyler, (TV: Aliens of London) Martha Jones, (TV: 42) Jack Harkness, (TV: The Parting of the Ways) and Donna Noble. (TV: The Sontaran Stratagem) Martha and Jack's keys were modified into perception filters. (TV: The Sound of Drums)
 * The Claw Shansheeth attempted to conjure a copy of the Doctor's TARDIS key through use of a memory weave on Sarah Jane and Jo Jones' minds. Sarah Jane and Jo overloaded the device and the partially created key disappeared. (TV: Death of the Doctor)
 * Clara Oswin Oswald received a copy of the key, although she lost it when the ice sculpture governess pulled her out of the TARDIS. However, the Doctor returned it to her while she was dying. (TV: The Snowmen)
 * At some point farther into their adventures than with other companions, the Doctor gave Clara Oswald a key to the TARDIS. (TV: The Time of the Doctor)
 * The Twelfth Doctor had seven keys in his possession, hidden in different places inside his TARDIS. Clara attempted to steal them and blackmail the Doctor into saving Danny Pink. (TV: Dark Water)

Other TARDIS Keys
The Chancellery Guard on Gallifrey had cypher indent keys, which were TARDIS master keys of varying shapes under the control of the Castellan's office. Some of these worked by being inserted into the keyhole, some worked by simply being held near the lock. (TV: The Deadly Assassin, The Invasion of Time)

Behind the scenes

 * The original spade-type TARDIS key was designed by Jon Pertwee and began use in Season 11.
 * The Rassilon-style key was designed by Mike Tucker.
 * The prop used for the TV movie was a BBC-licenced replica of the classic spade key, sold at the time by now-defunct American memorabilia catalogue 800-TREKKER.
 * The 2005 key was a standard nightlatch key made by UK lock manufacturer ERA, and it was the real key to the working lock on the TARDIS prop. Spares were stored in a secret compartment inside the TARDIS prop.