Inner door

The inner door was the second of the two doors of the TARDIS, as used by the First Doctor, following the outer door. The inner door was at the rear end of the exterior entrance chamber, leading to a fourth dimension which began with the control room. As the Doctor noted to Ian Chesterton and Barbara Wright, a reversal of time acceleration occurred at the inner door, explaining how the TARDIS was bigger on the inside. (COMIC: The Secrets of the Tardis)

When the TARDIS's fault locator became stuck on, the TARDIS attempted to warn its crew by, among other things, opening and closing the inner doors. (TV: The Edge of Destruction)

The inner doors later suddenly opening while in flight caused the TARDIS and its crew to be miniaturised. (TV: Planet of Giants)