The original TARDIS key was designed by Jon Pertwee during his years as The 3rd Doctor. It was his feeling that the TARDIS should have a unique key. This design was used during the Tom Baker and Peter Davison years as well.
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TARDIS Key Reproduction
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Back of Key
Most fans believe that the image on the back of the key represents the constellation Kasterborous in which the Time Lord's home planet of Gallifrey is located. Later in the series, during the Colin Baker and Sylvester McCoy years an alternate key design was used featuring the Seal Of Rassilon. Producers reverted to the original key design for the 1996 movie. Beginning in 2005, the new series has featured a simple Yale lock key instead of the more elaborate designs of the past.