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 which could be customised by the owners. 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)

Because of a metabolism detector behind the lock, (TV: Spearhead from Space) unauthorised people who stole the Doctor's TARDIS key usually found themselves unable to open the TARDIS doors without the Doctor there. (AUDIO: The High Price of Parking, TV: Spearhead from Space) Nevertheless, companions were often able to use the Doctor's key without the Doctor around, (AUDIO: The Jabari Countdown, Lucie Miller) and many had their own keys to the TARDIS. (TV: The Sensorites, Aliens of London, et al.)

The keys changed shape, just as the TARDIS did. (AUDIO: The Alchemists)

Others
The Doctor usually had their TARDIS key on their person at any given time. There were numerous other known copies of that key.

A spare key was 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 Ordeal") Susan's second key was seen when Susan locked the TARDIS aboard a human spaceship in proximity of the Sense Sphere. (TV: "Strangers in Space") 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 was picked up by Ben Jackson and he was able to get inside the TARDIS with Polly Wright when they tried to return it to the Doctor. (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)

Charlotte Pollard had a TARDIS key. (AUDIO: Time Works)

The Eighth Doctor intended to give Lucie Miller a TARDIS key with a Christmas-themed musical ringtone as a Christmas present. (AUDIO: Relative Dimensions)

Helen Sinclair had a TARDIS key by the time of the TARDIS's restoration. (AUDIO: Dead Time)

Sarah Jane Smith took a double-helix shaped key, apparently made of plastic, with her when she left the TARDIS. (PROSE: Interference - Book Two)

Anji Kapoor was given a Yale design key, (PROSE: The Crooked World) as was Rose Tyler, near the start of her travels. (TV: Aliens of London) Jack Harkness's key, (TV: The Parting of the Ways) Martha Jones' key (TV: 42) and Donna Noble's key (TV: The Sontaran Stratagem) were of the same design. Martha and Jack's keys were briefly modified into perception filters. (TV: The Sound of Drums)

The Ninth Doctor kept at least one copy of his key, the one he gave to Rose Tyler, on a long metal chain with a D-shackle at the end. (TV: Aliens of London) Adam Mitchell was briefly in possession of Rose's copy of the key, which was nearly stolen by the Editor. This further proved to the Doctor that Adam was unfit for companion life. (TV: The Long Game) The Tenth Doctor opted to keep one on a cord of twine that the Weeping Angels stole off him while transporting him to 1969. (TV: Blink) The Eleventh Doctor, by the end of his life, wore a copy on a cord around his neck, which he gave up to Tasha Lem. When he and Clara needed to escape the Weeping Angels on Trenzalore, he produced another key that he kept underneath his wig to summon the TARDIS around the two- "the old 'key in the quiff' routine." (TV: The Time of the Doctor)

A spade key was worn on a choker by Romana II. (WC: Shada)

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 the original Clara Oswald a key to the TARDIS. (TV: The Time of the Doctor)

To Clara's knowledge 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, but the Doctor outwitted her and helped her regardless. (TV: Dark Water) At this point in time she did not appear to have her own key, as none was produced by her during her blackmail attempt; she subsequently reacquired a key (TV: Sleep No More) which she still possessed after her extraction from the trap street as she was later able to enter the Doctor's TARDIS before returning it to him. (TV: Hell Bent)

River Song was in possession of a TARDIS key, which she frequently used to steal away the TARDIS for her own adventures without the Doctor's knowledge. (TV: The Husbands of River Song)

Cypher indent 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. Cypher indent keys were specific for each model of TARDIS. (TV: The Deadly Assassin, The Invasion of Time)

Behind the scenes

 * 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.