Ye Olde Cobbler used a measure called the “Iron” or “°” to measure the thickness (ie. 12°) of the materials he used to repair shoes.
Europe changed to the metric system and so their manufacturers started referring to their products with a Milimeter measurement (ie. 9mm).
In America, where people usually picked up how to do stuff and weren’t formally trained, stuck to fractions of inches (ie. ¼”) to measure thickness.
I made this chart as a scale to put on various repair publications. It makes a comparison between inches, iron and milimeters. The image above is not to scale. The actual size would be the same as the chart would imply: 1 inch.