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Yeah, these examples are a point of reference. As in a two by four. In the real world a 2x4 is actually 1 3/4 x 3 3/4 IIRC. Just a point of reference so we can all be on the same page.

Not sure when the metric system came to Europe. I'm guessing ....... just guessing, the inch system was universal back then, and using eighths and sixteenths as a standard was convenient.