Putting two spaces between sentences leaves a document riddled with holes. Aesthetic speed bumps, if you will. They are distracting to the eye and disrupt the flow of the narrative. Even to people who aren’t grammar nerds like me.
I immediately notice the extra space – it looks vacant, haphazard, like the following sentence broke free of its tether and is swinging madly in the wind. So, I was quite pleased to find that Farhad Manjoo of Slate feels the same way. And has let the world know (with puns and spaceship allusions, which make everything better). I might have done a fist pump or two.
I don’t care what you might have been taught in grade school. It’s once space after a period. Commas go inside quotation marks. A semi colon cannot be used instead of a comma if there seem to be too many commas. And I will continue poking at your paper with my purple pen – since red seems too angry – until you get it right. After which I shall retreat into my dusty, dictionary-filled lair. At least until the next inquiring grammarian comes along.
Check out the explanation for one space here: Space Invaders