Paris point

The Paris point is a unit of length defined as 2/3 cm. It is commonly used for shoe sizes in Continental Europe.

The unit was invented by French shoemakers in the early 1800s. Its origin probably lies in $2/3$ centimetre being very close to $2/3$ inch; a French inch pouce-roi is around 27 mm, a quarter of that is 6.7 mm, close to 6. mm defined for the Paris point.