Years ago there were many screw thread and spanner sizes, Whitworth, BSF, AF etc then some bright spark introduced metric as well just to confuse us and bulk out our toolboxes.
http://www.vars.freewire.co.uk/tech/charts/spanners.htm
http://www.boltscience.com/pages/screw1.htm
Another long forgotten problem was hitching up a trailer with a 2” coupling to a vehicle with a 50mm tow ball.
Any Imperial v’s Metric stories