system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the purity of precious metals measured in karats rather than just using percentage?
gpt
Carats were/are a measure of weight, not purity. In olden times they thought carob seeds didn't vary in weight much, so they used them on balance scales as a unit to measure weight. At some point, there was a gold coin (there's debate about which, though most think it was a Roman solidus) that was pure gold and weighed 24 carob seeds/carats. Pure gold came to be known as 24 carat gold. If you mixed in other metals, you'd reduce by the number of carats, so 50%gold would be 12 carat.

In the US, to lower confusion between carat as weight and carat as purity, they agreed to spell it karat when applying to purity. Otherwise, is just a term that's been around a long time.