Roundel

A roundel was a key feature of the Doctor's TARDIS. Roundels were mainly used as a decorative feature, although occasionally they did actually fulfil a function.

The First Doctor's TARDIS had basic white roundels on the wall, circular in shape. They were indented into the wallpaper. (TV: An Unearthly Child)

The Third Doctor added plastic furnishings to the edges of the roundels. One of them then became the replacement for the scanner. (TV: Day of the Daleks, The Curse of Peladon) Later, his TARDIS had reverted to its original design. (TV: The Three Doctors)

Some of these roundels doubled as access covers to circuitry in the walls, at least during the Fifth Doctor's tenure. (TV: Terminus)

The Eighth Doctor's TARDIS had circular cut-outs in the support beams. (TV: Doctor Who)

In the Ninth Doctor's first control room, the walls of the TARDIS featured backlit hexagons. (TV: Rose). By the time of the Tenth Doctor, overhead lights added to the TARDIS's illumination, so the roundels fulfilled a mainly decorative function.

The first of the Eleventh Doctor's TARDISes had not so many roundels in the walls, but the theme did continue slightly, and there was a larger circular screen set into the wall near the door. One of his console rooms had blue and red neon circles. (TV: The Snowmen)