On Tue, 28 Apr 2020 09:04:47 +0000, Warren Boyd wrote:
Two spaces in a formal document, letter, novel. It
readability, even with proportional fonts.
Professional-quality typesetting includes different-width spaces,
including an en-space and an em-space. The em-space, the slightly
longer one, is commonly used after a full stop. It is not twice as wide
as an en-space.