Paris point

The Paris point is a unit of length defined as ⅔ of a centimetre (6.6 mm or ~0.26 in). It is commonly used for shoe sizes in Continental Europe.

Its origin probably lies in 2/3 of a centimeter being very close to a quarter of an inch ; a French inch "pouce-roi" is around 27mm, a quarter of that is 6.7mm, close to 6.6 mm defined for the Paris point.