The switch to Metric in the US was made easier by a personal decision of Carl Johansson... (backed up by wikipedia)[1] In the early 20th century, the U.S. in

The switch to Metric in the US was made easier by a personal decision of Carl Johansson... (backed up by wikipedia)[1] In the early 20th century, the U.S. inch was effectively defined as 25.4000508 mm (with a reference temperature of 68 °F (20 °C)) and the U.K. inch at 25.399977 mm (with a reference temperature of 62 °F (17 °C)). When Johansson started manufacturing gauge blocks in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 °C (68 °F), accurate to within a few parts per million of both official definitions. Because Johansson's blocks were so popular, his blocks became the de facto standard for manufacturers internationally, with other manufacturers of gauge blocks following Johansson's definition by producing blocks designed to be equivalent to his. [1] https://en.wikipedia.org/wiki/Gauge_block https://en.wikipedia.org/wiki/Gauge_block