Metric vs. other measurement systems - chart

[Moby Duck]

and this means what??? nothing at all. the unit could be the width of a wall nut. and that would work just fine as long as it was a std nut size.

Except when you try to screw your base wall nut nut to the international space station, it’s not going to fit. They have already tried that, and failed.
For those in doubt, BSW is not British Standard Wall-Nut.