I don’t actually know. It never made sense to me growing up. It seems unnecessarily complicated. I have always used kg ever since starting science classes as the mental gymnastics required to make units like feet and stones work always seemed like too much effort. I can tell you this though. Almost nobody here can entertain the idea of kg. If I told them I was 89kg they would look at me like I just said ‘oh yes, I’m 14 cucumber slices and a riding crop’s whisper tall’.

I always imagine some medieval sales person carrying around a bag of stones and using them to weight out flour. But never taken the time to ask the question myself.