system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are screens measured diagonally?
gpt
The first CRT screens used a circular glass bulb as a display, with a rectangular image displayed on it, and the diameter of the circle was given as the size of the screen.  When rectangular CRT bulbs were developed, the diagonal size was used because it was the closest equivalent measurement to the existing circular bulbs.  For a long time, TV screens were all 4:3 ratio with a standard number of vertical lines; so there was little point to changing the measurement standard.