Per Mille Sign

Look closely at the percent sign, then add a zero to the bottom half: that is the whole design. Per mille means parts per thousand, from the Latin per mille, and where a percentage divides by a hundred this one divides by a thousand. Oceanographers use it for salinity — seawater runs about 35‰ — and European railways use it for gradients, where 50‰ describes a five percent climb.

The place it turns up most in daily life is blood alcohol. Unicode's own notes on the character cite that use, and it is no accident: promille is a native word in German, Dutch, Danish, Norwegian, Swedish and Finnish, so a 0.5‰ limit reads naturally in those languages in a way that "nought point nought five percent" never would.

One correction worth making, because this symbol gets dragged into finance on a misunderstanding: a basis point is not a per mille. A basis point is a hundredth of a percentage point — one ten-thousandth — which makes a single per mille equal to ten basis points. The character that actually lines up with the basis point is per ten thousand, which adds one more zero again, and Unicode's own annotation on that one is refreshingly blunt: rarely used. Where per mille does earn its keep in money is older and quieter — insurance rates quoted per thousand of insured value, and property tax mill rates.