According to this Slate article “Space Invaders: Why you should never, ever use two spaces after a period“:
Typing two spaces after a period is totally, completely, utterly, and inarguably wrong.
More from the article:
Ilene Strizver, who runs a typographic consulting firm The Type Studio, once wrote. “When I see two spaces I shake my head and I go, Aye yay yay,” she told me. “I talk about ‘type crimes’ often, and in terms of what you can do wrong, this one deserves life imprisonment. It’s a pure sign of amateur typography.”
MP: I confess that: 1) I wasn’t even aware of this issue, and 2) I’m totally “old school” on this rule, probably because I learned how to type on a manual typewriter many years ago and was taught the “two space rule.”
HT: Paul Sebastian