Doctor Who’s TARDIS key has changed quite a bit over time. Much like the Doctor himself, both the TARDIS and its key are constantly evolving. The acronym stands for "Time And Relative Dimension In Space”, and it's this machine that enables the Time Lords to travel through time.

If the TARDIS is still functioning properly it will blend in with its surroundings, regardless of where it may be. However, the chameleon circuit used in the Doctor's TARDIS was busted long ago, making it permanently appear as a blue police box. However, just because it looks like a police box, doesn’t mean it can be accessed like one. The Doctor still needs a special key to open it.

Related: Doctor Who? The Answer To The First Question Explained

Terrasolo Pictures (via io9) charted all of the changes the TARDIS key has been through from 1963-2013. The collection even includes the keys of companions and friends. Some of the keys are more accurate than others, depending on how many onscreen details the artist was able to get. The original was a simple Yale key but by the Third Doctor, the shape had changed significantly. It was during this time the Doctor had begun experimenting with his TARDIS keys, though he would eventually settle on the spade shape. The Fourth Doctor got more radical, even making a plastic double-helix key - though that’s not shown here. He did eventually return to the spade shape as well, and later, the Yale key. The Fifth and Sixth Doctors used the very same.

Doctor's Who's TARDIS Keys

It was the Seventh Doctor who really changed it up though. He gave the key a sort of fan shape and incorporated the Seal of Rassilon, a symbol used by the Time Lords. Following that, Doctor Who returned to the spade shape, which the Eighth Doctor continued to use as well. The Eighth Doctor later started using the Yale-type keys, which the Ninth, Tenth, and Eleventh all used as well.

Although the main function of these keys is to unlock the Doctor’s TARDIS, they have performed other functions as well. For example, they can open other TARDISes. The keys can also glow and get warm when a TARDIS is about to show up. Some keys have been capable of remotely locking or even tracking and locating the TARDIS. They are also quite resilient, though they have been destroyed by lava. Many of the Doctors kept multiple keys at a time, with the Twelfth having seven of them. The TARDIS key is an undeniably handy tool, right up there with Doctor Who's Sonic Screwdriver. It's safe to say the Doctor wouldn't be traversing time without one.

Next: Doctor Who: River Song's Timeline Explained