Real world interface

The real world interface, on some occasions also referred to as the door, is the part of the TARDIS which connects the outer plasmic shell of the ship to the interior.

Functions
By crossing the dimensional threshold of the interface a person or object underwent an instantaneous translation from the ongoing space-time continuum occurring outside the vessel to that pertaining in the separate micro-environment within (or vice versa). This process was known as going into the TARDIS or coming out of the TARDIS. (DW: Logopolis)


 * Depending on how the chameleon circuit configured the TARDIS, the real world interface might actually resemble doors or a doorway. On other occasions, the real world interface might not appear visibly at all, making the TARDIS appear to have no obvious entrance or exit.

Opening the doors while the Doctor's Type 40 was in transit usually had grave consequences (DW: Planet of Giants, Warriors' Gate), but once materialised the TARDIS force field usually protected the interior even if the doors were opened on a hostile environment. If the doors were opened on a vacuum the interior of the ship would not undergo explosive decompression, for instance. (DW: Four to Doomsday, The Runaway Bride)

Real world interface of the Doctor's TARDIS
The Doctor's TARDIS originally had what appeared to be two sets of doors, the police box doors of the outer shell, which were manually operated from both within and without, and a pair of more substantial doors covered on both sides in the usual roundel motif. The inner doors were usually power-operated - operating a switch on the console or opening the outer doors manually caused the inner doors to open automatically. A dark void linked the two sets of doors. The pressure exerted by the door servos was not especially great: a Cyberman appeared able to force the doors open against the mechanism. In the event of a total power failure the doors could be cranked open and shut manually. (DW: Death to the Daleks, Full Circle (Novelisation), Earthshock)

By the time he met Rose Tyler in his ninth incarnation, the Doctor's TARDIS had a simpler arrangement, with the police box doors of the ship opening directly onto the console room. The doors were always manually operated now.