Good to know! I am reminded of using *roff in the 1980s. Punctuation such as periods at the end of lines marked end of sentences, and nroff would insert two spaces in that case.

  In HN, you can force two spaces between
  sentences.  But only in code blocks.

Because I learned to type on a typewriter, I always ended every sentence with a period and two spaces. But LaTeX was kind enough to not care whether I put one or two spaces, and gave me the right proportional spacing regardless.

Thanks for sharing. I'm a double spacer, but have to agree it does look to wide most of the time, while single-spacing looks too narrow. 1.5 sounds like a greatGoldilocks spacing. I only wish it were easier to use.

> Two ('full') space characters were added because of (tele)typewriters and their fixed width fonts

I could never agree with this, because monospace fonts are already adding extra space with the dot character, which is much narrower in proportional fonts. That fact alone makes the visual gap already similarly wide as it would be in typeset proportional text. Adding a second space makes it much too wide visually (almost three positions wide). It looks like badly typeset justified text.

(I understand why people are doing it, I just don’t agree on aesthetic grounds.)