TARDIS key

The 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
The key worked like a normal key would, 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 larger than the keyhole.

Design
The key to The Doctor's TARDIS has been seen in several forms. The First and Second Doctors used a key which resembled a plain Earth Yale lock, such as might be used with a genuine police box. The Third Doctor started out with the same key (DW: 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, 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. (EDA: 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. (DW: Ghost Light)

The Seventh Doctor eventually went back to the spade key, which after regenerating the Eighth Doctor continued to use. (DW: Doctor Who)

The Ninth and Tenth Doctors reverted to the Yale key.

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. (DW: The Mark of the Rani)
 * The TARDIS key glows and grows warm when the TARDIS is about to materialise nearby. (DW: Aliens of London)
 * 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. (DW: Father's Day)
 * If modified properly, the TARDIS keys retained perception filter properties of the TARDIS. (DW: The Sound of Drums)
 * The key could also be used to "remotely" lock the TARDIS from a distance, similar to a car alarm. (DW: The End of Time)
 * The key could also be used to place the TARDIS one second out of synch with the rest of the universe, preventing it from being seen. (DW: The End of Time)

Known Copies

 * The Doctor's Key
 * Susan's Key, left in the dirt on the bank of the River Thames after the Daleks invaded Earth in the 2160s. (DW: The Dalek Invasion of Earth)
 * The key that fell from the Doctor's cloak and was picked up by Ben Jackson. (DW: The War Machines)
 * A spare copy of the key kept by the Seventh and Eighth Doctors in a secret compartment in the TARDIS' outer shell (DW: Doctor Who (1996))
 * A plastic double-helix shaped key Sarah Jane Smith took with her when she left the TARDIS. (EDA: Interference - Book Two)
 * A copy of the spade key worn on a choker by President Romana (WC: Shada)
 * The keys given to Rose Tyler, Martha Jones, Jack Harkness, and Donna Noble.

Other TARDIS Keys
The design and abilities of keys to TARDISes other than the Doctor's remain unknown.

The Chancellery Guard on Gallifrey had "cipher indent keys," master TARDIS keys of varying shapes. Some of these worked by being inserted into the keyhole, some worked by simply being held near the lock. (DW: The Deadly Assassin, DW: The Invasion of Time)

Behind the Scenes

 * The original spade-type TARDIS key was designed by Jon Pertwee.
 * The Rassilon-style key was designed by Mike Tucker.
 * The prop used for the TV movie was a BBC-licenced replica sold at the time by now-defunct American memorabilia catalog 800-TREKKER.
 * The 2005 key was a standard door lock key made by UK lock manufacturer ERA. Spares were stored in a secret compartment inside the TARDIS prop.