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
Entry to the Doctor's TARDIS was allowed by inserting a key into the lock, just like a real police box. However, the locking mechanism did not respond to police-issued keys, and would not even open when unauthorized persons used the Doctor's keys. The Brigadier was prevented from opening the TARDIS in the Doctor's absence, because of the metabolism detector on the lock. (DW: 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. (DW: Doctor Who, The War Machines, Blink)

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 lock.

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. (DW: The Dalek Invasion of Earth, Utopia) At one point, the Doctor installed 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. (DW: The End of Time)

It was unclear whether the lock automatically secured from the exterior however. There were occasions on which the Doctor or his companions needed to use the key to lock the doors (DW: The Sensorites) but others (DW: Spearhead from Space, The Christmas Invasion) in which the act of merely closing the doors locked the TARDIS. It was possible that the lock could be set secure either automatically or manually.

However, a key is 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. (DW: Silence in the Library/Forest of the Dead) After regenerating into his eleventh self, the Doctor again opened the TARDIS by snapping his fingers, demonstrating that this method had become a reliable way for the him to do so. (DW: The Eleventh Hour)

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 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. (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 the Eighth Doctor initially continued to use. (DW: Doctor Who) At some point, the Eighth Doctor returned to using the Yale key. (EDA: The Crooked World, History 101)

The Ninth, Tenth, and Eleventh Doctors used Yale-type keys, now 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. (DW: The Mark of the Rani)
 * The TARDIS key glowed and grew warm when the TARDIS was about to materialise nearby (DW: Aliens of London) or when an incapacitated TARDIS had finished rebuilding itself. (DW: The Eleventh Hour)
 * 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 remotely lock the TARDIS from a distance, similar to a car alarm. (DW: The End of Time)
 * The key could 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)
 * 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. (IDW: The Forgotten)

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 Doctor to replace one Marco Polo had taken away. He later recovered both keys. (DW: Marco Polo)
 * Susan's first key, destroyed by the Doctor whilst shorting out static electricity production in the Dalek City. Just before he destroyed it, he claimed he'd make her a new one.  (DW: The Daleks)
 * Susan's second key was definitively seen when Susan locked the TARDIS aboard a human spaceship in proximity of the Sense Sphere. (DW: 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. (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 spade-type key kept by the Seventh and Eighth Doctors in a secret compartment in the TARDIS' outer shell (DW: Doctor Who)
 * A double-helix shaped key, apparently made of plastic, which Sarah Jane Smith took with her when she left the TARDIS. (EDA: Interference - Book Two)
 * Anji Kapoor's key, which was of the Yale design. (EDA: The Crooked World)
 * A spade key worn on a choker by President Romana (WC: Shada)
 * The Yale-type keys given to Rose Tyler, Martha Jones, Jack Harkness, and Donna Noble. Jones' and Harkness' keys were modified into perception filters; it was unclear whether they were ever returned to their original state of being simple keys.  Although the times when Tyler, Jones and Noble were given keys were well established (DW: Aliens of London, 42 and The Sontaran Strategem), it ws unrecorded when Jack was given his.

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, The Invasion of Time)

The Claw Shansheeth attempted to conjure a TARDIS key through use of a Memory Weave. The device overloaded and the partially created key dissapeared. (SJA: Death of the Doctor)

Despite the unique properties of the Doctor's TARDIS key, "master" keys did exist back on Gallifrey. These keys, under the control of the Castellan's office, could open any TARDIS, including the Doctor's. (DW: 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 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.