Every major style guide agrees on a single space after a period. But most of us were taught to type two spaces after a period. Why?
Blame the manual typewriter.
Quick typography lesson: Unlike printing presses and computers, which have proportional fonts like the one on this blog, manual typewriters only have monospaced fonts (see a sample of monospaced fonts), where every character has the same width.
To make up for the manual typewriter’s shortcomings, typing schools taught an entire generation the double-space rule. And the bad habit stuck, even when computers added proportional fonts in the 1970s.
Your computer is not a typewriter. It has proportional fonts. Stop typing two spaces after periods. Just stop it.
Hat tip to Farhad Manjoo at Slate for the history lesson.