Roundel

A roundel (TV: Terminus) was a key standard feature of all TARDISes. (TV: Hell Bent) Roundels were mainly used as a decorative feature, although occasionally they did actually fulfill a function.

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

The Third Doctor added plastic furnishings to the edges of the roundels. One of them then served the role of the scanner. (TV: Day of the Daleks, The Curse of Peladon, The Time Monster) Later, his TARDIS reverted to its previous design. (TV: The Three Doctors)

At least some of these roundels doubled as access covers to circuitry in the walls. (TV: Terminus, Attack of the Cybermen) Some others were shown to be small storage units. (TV: Castrovalva)

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

In the Ninth Doctor's control room, the walls of the TARDIS for a time featured backlit hexagons. (TV: Rose) By the time of the Tenth Doctor, overhead lights were added to the TARDIS' illumination, so the roundels fulfilled a mainly decorative function. (TV: The Runaway Bride) The first of the Eleventh Doctor's TARDIS console room "desktops" 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. (TV: The Eleventh Hour) Another 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 and reverted to the War Doctor's desktop settings, 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)

The Twelfth Doctor made a minor redecoration to his TARDIS when he added lit-up roundels to the walls, noting that he used to have a lot of "round things" and feeling partly unsatisfied with the lack of more. (TV: Deep Breath) He eventually did increase the number of roundels featured in his control room, specifically on the lower floors. (TV: Under the Lake)

At some point, the Twelfth Doctor's TARDIS resumed the use of some of the roundels as storage units. One such example was a hiding space for River Song's alcohol. This came as much of a surprise to the Doctor. (TV: The Husbands of River Song)

Behind the scenes

 * Though a common enough word in fandom, roundel is a tough word to find in a televised script. However, it has happened, perhaps most clearly in Terminus, where Turlough, Tegan and the Black Guardian use the word several times at the top of the first episode.
 * A scene recorded for The Awakening part one, but edited out of the finished programme due to the episode overrunning, featured Tegan encountering Kamelion with his hand inserted into a roundel in a corridor, learning more about the TARDIS.