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 fulfill 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. Later, his TARDIS had reverted to its original design. (TV: The Three Doctors)

Someone drew roundels on two of the walls in the Fifth Doctor's house. (PROSE: The Comet's Tail)

In the Ninth Doctor's first control room, the walls of the TARDIS featured backlit hexagons. (TV: Rose)