Blackboard

The Doctor had a blackboard in his TARDIS. The Fourth Doctor used a blackboard to teach Leela how to write. (TV: The Invisible Enemy) In his new control room, the Twelfth Doctor used to write undecifrable calculations over his blackboard. (TV: Into the Dalek onwards)

When Rubeish wrote equations on the TARDIS with a piece of chalk, the Third Doctor reproached him because the TARDIS was not a blackboard. (TV: The Time Warrior)