system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we write the letter "A" like an "O" with a tail but type it like an backwards "6"?
gpt
Easier visible differentiation. 

The small letter "a" like in this font is much easier to tell apart from an "o" than the "O|" combined shape. Consider the words "ball" and "boll". If the printing press (the precursor of the typewriter) had any blurring at all in its ink, you wouldn't be able to tell which was which if the ink ran the "a" and the "l" together in the first word. Ditto oh/ah; fob/fab; loon/loan and so on.

But it's hard to write an "a" while continuing to the rest of the letters in the word without lifting the pen, so we write the O| version instead. We do the same while writing the letter k, putting a loop in the top right leg so we can continue onto the rest of the word without slowing down to reposition the pen.