The dangerous bend or cautionsymbol ☡ ( U+2621 ☡ CAUTION SIGN ) was created by the Nicolas Bourbakigroup of mathematicians and appears in the margins of mathematics books written by the group. It resembles aroad 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.
Others have used variations of the symbol in their textbooks, and 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.