The writer explains the origin of the scale measuring the external diameter of different types of bougies, catheter or probes. That scale has turned out to be an essential tool for the best of several specialities created in the 19th century, namely urology. An instrument maker close to the surgeons whose ideas he would quickly grasp, Joseph F. B. Charriere, proposed a scale with thirty holes that would make it possible to get external gauge of the third of a millimetre. Several people tried to imitate his device but his scale was eventually recognized as the most efficient one all over Europe, accepted even by the British and then the Americans. Its usefullness goes on to this day as the French scale has become universal together with the unit that it entails, the Charrire unit, which is still the reference to measure all the diameters used by surgeons and doctors.