Question mark

The question mark, ?, was a piece of punctuation used by many languages on Earth to denote an interrogative.

As Fashion for The Doctor:

During some of his incarnations, the Doctor appeared to adopt the question mark as something of a personal motif. The Fourth, Fifth, and Sixth Doctors displayed question marks on the collars of their shirts. (DW: The Leisure Hive, Castrovalva, The Twin Dilemma, Time and the Rani) The Seventh Doctor wore a jumper covered with the symbol, carried an umbrella with a question-mark-shaped handle, carried a business card with a stylized question mark, and when asked to sign his name, did so with a question mark. (DW: Remembrance of the Daleks)

It was unclear whether the Doctor regarded the symbol as a question mark or as an unknown Gallifreyan symbol. Neither he nor any companion ever explicitly referred to the motif as a question mark.