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)

When the Tenth Doctor's TARDIS interior glitched due to the presence of the War Doctor and the Eleventh Doctor, the Eleventh Doctor pointed out the "round things" to his predecessor. The Tenth Doctor voiced his love for the "round things", though he admitted he had no idea what they were when his successor asked. (TV: The Day of the Doctor)