Bourbaki dangerous bend symbol

The dangerous bend or caution symbol ☡ was created by the Nicolas Bourbaki group of mathematicians and  appears in the margins of mathematics books written by the group. It resembles a road sign that indicates a "dangerous bend" in the road ahead, and is used to mark passages tricky on a first reading or with an especially difficult argument.

Variations
Others have used variations of the symbol in their books. The computer scientist Donald Knuth introduced an American-style road sign depiction in his Metafont and TeX systems, with a pair of adjacent signs indicating doubly dangerous passages.

Typography


In the LaTeX typesetting system, Knuth's dangerous bend symbol can be produced by first loading the font manfnt (a font with extra symbols used in Knuth's TeX manual) with   and then typing

There are several variations given by,  ,  ,  , and.

In popular culture
The Twenty One Pilots song "Overcompensate", makes direct reference to the symbol. Since 2018, the Nicolas Bourbaki group has also been referenced in the conceptual narrative of the band.