William James wrote:

> TeX and LaTeX, for example, quite properly put extra space
> after the end of a sentence.  Since what we type here will
> usually be displayed monospaced, a sensible person who is
> trying to make his message as readable as possible will put
> two spaces between sentences.

Two spaces are needed even when the posts are seen in
a proportional font; without them, there is no extra space
between sentences.