 For 21 of the 26 season of Dr Who the key used for the TARDIS was a  simple Yale lock key and was used in the normal way. The unique key  used in this film was devised by Jon Pertwee (qv) and was used for  his last three years in the show and was also used by  Tom Baker (I) (qv)s Dr for his first two seasons. For the first  time viewers are shown exactly how this version of the key works.