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 his fourth, fifth, sixth and seventh incarnations, the Doctor adopted the question mark as something of a personal motif. However, 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.

Nevertheless, the fourth, fifth and sixth incarnations all displayed what appeared to be question marks on the collars of their shirts. The seventh incarnation wore a jumper covered with the symbol, carried an umbrella with a handle shaped like a question mark, 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)(DW: The Twin Dilemma)(DW: Castrovalva) (DW: Logopolis')