system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are UltraWide monitors 21:9 and not 7:3?
gpt
First, it's not actually 21:9; it's 64:27.  21:9 is just a close approximation that can be easily compared to 16:9.

As to why it's 64:27, it's because monitors were initially in 4:3 ratio.  Wide screen squared the ratio (4:3 ⨯ 4:3) to 16:9 and ultrawide screen cubes it (4:3 ⨯ 4:3 ⨯ 4:3) to 64:27.