>I use single spaces between sentences, but my ancestors
>used 2... who knows why? :).

Typewriters.

In typesetting, especially when doing right-margin justification,
we have "stretchy spaces" between words.  The space after end-of-
sentence punctuation marks is supposed to be about 50% larger than
the width of the between-words spaces, and if the word spaces get
stretched, so should the end-of-sentence space.  Note that this is
all in the variable-pitch font world.

Since typewriters are fixed-pitch, the way to emulate the
1.5-space-wide gap is to expand it to 2.

Chris