From Farhad Manjoo at Slate.
Do you use one space or two after a period? Typographers recommend a single space. I confess that I use two.
The reason for the two-space convention is an interesting example of how technology changes behavior.
The problem with typewriters was that they used monospaced type—that is, every character occupied an equal amount of horizontal space. This bucked a long tradition of proportional typesetting, in which skinny characters (like I or 1) were given less space than fat ones (like W or M). Monospaced type gives you text that looks “loose” and uneven; there’s a lot of white space between characters and words, so it’s more difficult to spot the spaces between sentences immediately. Hence the adoption of the two-space rule—on a typewriter, an extra space after a sentence makes text easier to read.