Despite what countless people seem to think, it’s “totally, completely, utterly, and inarguably wrong” to use two spaces after a period, writes Farhad Manjoo in Slate. It’s been the case since the early 20th century, but you wouldn't know it based on his readers ("In editing letters for my occasional tech-advice column, I've removed enough extra spaces to fill my forthcoming volume of melancholy epic poetry, The Emptiness Within") or his friends: When he asked a dinner party group consisting of doctors and computer programmers, "Everyone—everyone!—said it was proper to use two spaces." When he told them they were doing it wrong, they had one big question: Says who?
In one word: Typographers. It's a "canonical" rule of the profession, "in the same way that waiters know that the salad fork goes to the left of the dinner fork." So why do we insert spaces to our heart's delight? Blame manual typewriters, which used monospaced type—each letter was afforded the same amount of space. On those machines, two spaces after a period made things more readable. But nearly every font on the modern PC is proportional: an "I" gets less space than an "N," for example. Now, "adding two spaces after a period no longer enhances readability, typographers say. It diminishes it."
(Read more typing stories.)