" At First, these wristwatches ended up just one-off or customized creations That usually appeared a lot similar to the pocket watches from which they developed. But before long, wristwatches took on their own aesthetic, with their own individual technical innovations to unravel new issues offered by wrist-borne timekeepers. Weiss https://www.watchesworld.com/a-history-of-watch-reference-numbers/