Used from the beginning of the nineteenth century in the time of Napoleon and extended to the present day, in Europe this is equivalent to 2/3 centimeter, which would be about 6,667 millimeters.
That is, a shoe size 42 measures 28 cm, these centimeters correspond to the measurement of the footwear, not the foot, understanding that there must be some free centimeters between the longest foot (the two feet of a person have different measurements) and the edge of the footwear.
In addition the French dot is one of the measuring systems that admits the average sizes such as for example 42.5