system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do all eye doctors use the Big E as the top letter for their charts?

Always wondered why, and how they decided on that specific letter?
gpt
Because original eye charts (Snellen Chart) only had the letter E, but each letter would be rotated to point into a different direction (with the opening towards up, down, left or right), so that you can just point to which way the letter E is pointing (useful when some of your clients might be illiterate, too young to read, or non-English speakers). The E at the top is a reference to the old chart.