Spaced out

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. Here's the thing, though: Monospaced fonts went out in the 1970s. First electric typewriters and then computers began to offer people ways to create text using proportional fonts. Today nearly every font on your PC is proportional. (Courier is the one major exception.) Because we've all switched to modern fonts, adding two spaces after a period no longer enhances readability, typographers say. It diminishes it.

~"Space Invaders: Why You Should Never, Ever Use Two Spaces After a Period," by Farhad Manjoo