Caution Sign

It warns you that the reasoning ahead is easy to mishandle. In its best-known use, ☡ marks a tricky mathematical passage, a deceptive step, or an argument that deserves slower reading.

The mark was developed for Bourbaki's mathematical books. The group of mathematicians adopted the shared name beginning in 1935; its reader directions called the sign a tournant dangereux (“dangerous turn”). It flagged places where a reader might make a serious interpretive mistake.

Why is it called the dangerous bend symbol?

It borrows road-sign language for a difficult turn in reasoning. The bend is in the argument, not a physical road.

The idea travelled from Bourbaki's margins into technical books. In 1986, Donald Knuth used related road-sign artwork in materials for METAFONT, his font-making system, and TeX, his typesetting system. A doubled mark warned about especially obscure passages.

Should I use ☡ in a normal message?

Use it for a deliberately bookish warning; add words if stakes matter. Outside a math or typography context, ☡ can read as an obscure in-joke. It suits a difficult idea, a puzzle, or a playful warning. If someone needs to act for safety, spell out the danger.