River Song's TARDIS key

River Song held possession of a TARDIS key.

History
While searching for any of the first twelve incarnations of the Doctor she was aware of on Mendorax Dellora in 5345, River found the TARDIS though the Doctor himself was apparently absent. As such, River announced her intention to steal the TT Capsule. When one man, actually the Twelfth Doctor, protested that she could not steal "the box", River revealed that she had a key, proceeding to use it to enter the TARDIS. (TV: The Husbands of River Song)

On another occasion, River used the spare key in the cubbyhole above the "X" to borrow the Fourth Doctor's TARDIS. (AUDIO: Whodunnit?)