A tuning fork was a tool which aided the tuning of musical instruments.
The Second Doctor once used a tuning fork whilst trapped within a prison cell on the planet Ta in order to deactivate the audio lock that was being used to seal the exit. While the Doctor's efforts to find the correct sound proved unsuccessful, Jamie inadvertently wound up producing the correct sound when he threw the tuning fork at the cell wall in frustration. (TV: The Space Pirates [+]Robert Holmes, Doctor Who season 6 (BBC1, 1969).) Jamie would later call back to this event when the Doctor attempted to open a safe belonging to General Smythe, sarcastically asking him if he intended to pick the lock with a tuning fork. (TV: The War Games [+]Terrance Dicks and Malcolm Hulke, Doctor Who season 6 (BBC1, 1969).)
The Landlord used a tuning fork to summon the Dryads so they could devour students living in 11 Cardinal Road. (TV: Knock Knock [+]Mike Bartlett, Doctor Who series 10 (BBC One, 2017).)
The god of music Maestro used a tuning fork as a trigger to activate their powers. (TV: The Devil's Chord [+]Russell T Davies, Doctor Who series 14 (BBC One and Disney+, 2024).)
When the Doctor's TARDIS experienced turbulence from a time contour, the ship oscillated like a tuning fork. (PROSE: Time-Flight)