Thread by @gro_tsen

Following a(n old) comment by @JDHamkins on MathOverflow (‌mathoverflow.net/a/42998/17064‌), I realized something about the “dangerous bend” sign that's used by Donald Knuth to draw attention to difficult parts in a textbook. 🧵⤵️ •1/6

The Americans will think it's a European sign, and the Europeans will think it's an American sign. Why? Well, It's Complicated™. •2/6

The dangerous bend inside the sign was introduced by Bourbaki (and made its way into Unicode as U+2621 CAUTION SIGN; here's one: ☡). •3/6

But this is NOT what “dangerous bend” signs in contemporary European road signage look like. They look like this (this is the French sign, but most European signs should look pretty similar): •4/6

European countries, following the Vienna Convention on Road Signs and Signals use triangular shapes to denote danger signs. Furthermore, the Bourbaki sign is derived from the old version of this sign, which looked like this [source: commons.wikimedia.org/wiki/File:Born…‌]: •5/6

But Knuth took this (old French) “dangerous bend” symbol from Bourbaki and added an American-style warning sign around it, namely a yellow diamond. The closest actual American road sign seems to be this: •6/6

Slight correction: tweet 4/6 above, I should maybe have used this sign instead, which means “dangerous bends” rather than just “dangerous bend”. I'm not sure if the older signage had both versions or just one. But in any case, both shape and content differ. •7/(6+1)