Until 1959 the USAian inch was slightly more than 25.4 mm, the rest of the world-except Canada-used slightly less than 25.4 mm Canada used 25.4 mm. It was to do with the standards being based on lengths of brass (yards)and60 F or platinum alloy at "themelting point of ice" (metre) depending on whether you based the calculations on cooling down the brass or warming up the platinum. The only time it caused a practical problem was repairing large electronic valves on USN ships in British dockyards, the valves were designed in metric but made on USA machine tools, thus replacement parts were slightly too small when made on British machines. The difference was about 3.7 millionths of an inch per inch.