I grew up with both Imperial and Metric, so I guess that makes me bilingual
Most hand tools are in Imperial since the good stuff is either vintage British or vintage US. Then, tool slaving takes this a step further, with mortice chisels and plough plane blades being matched up. However, this is still not “measuring”, per se.
A turning point was the purchase of a Hammer A3-31 jointer-thicknesser/planer. This is Metric. Now machining boards is easier if done in Metric. This also affects grooves, since it is easier to thickness 6mm rather than 6.35mm (1/4”). And so there is a slow move to metric plough plane blades.
Metric will inevitably take over for measurements. This is not the same thing as measuring for parts. Measuring for parts does not need a number; instead it is required to be a good proportion, aesthetic, sympathetic. You do not get these from Metric or Imperial numbers. You don’t even measure them. Instead you determine them by play. And you transfer them practically, with a story stick.
Regards from Perth
Derek