Maybe there's been an unusual uptick in the frequency with which this issue is being addressed in my online circles by obnoxiously defensive and condescending people like this guy. Then again, maybe I just notice such discussions more readily than most. In any case, it's time for me to officially weigh in on a typographical issue that's been on my mind.
It's one of those things about which everyone seems to have a pretty resolute opinion, and there's a lot of needless anger flung in the faces of dissenters on both sides. The Single-Spacers chant the mantras of best practices and publishing industry standards while the Double-Spacers make emotional appeals to their preference for clarity and distinction. Designers rally in the name of even color and against the piercing disruption of dreaded white rivers, while even some of the most competent writers will wax poetic about the merits of "breathing room" in which to process the significance of the previous sentence before proceeding to the next.
The ever-amusing Bloom Country has cleverly and adorably addressed this question, albeit as a metaphor for the ways in which people so willingly and passionately divide themselves over innocuous issues.
First, just take a second to notice the clever touch of using two spaces after sentences spoken by the female character and only one after the male character's interjection in the last frame. Done? Cool, huh? OK, moving on...
Of course, Breathed is making a more high-minded point, but it applies literally to punctuation as well as it does to anything else. We choose to view issues as being about categorical rights and wrongs rather than just about preference or specific utility, and we develop the need for others to see what seems like the absolute truth in our own opinions, rather than acknowledging the existence of multiple or more nuanced truths.
There's no right number of spaces to put after a period—at least, not universally. Instead, there are different conventions that suit different purposes and people. If you like having two spaces at the ends of sentences, then by all means, please put two spaces after every sentence you ever write for your own consumption or for that of a friend or acquaintance who shares your preference. Put three spaces. When you're typing notes or an early draft, take all the breathing room and rest stops you need to stimulate your writing and comprehension muscles. But when you ship that novel off to the publisher, or when you're polishing up that cover letter or college essay, then it becomes necessary to demonstrate that you understand what contemporary writing best practices are, and that's when those extra spaces have to go. When you're a designer typesetting or a proofreader proofing, good form trumps personal whim. Apply the convention that suits your purpose.
Some people are bursting with empirical support for one practice or the other. Yes, current style manuals specify one space. Yes, older letterpress books sometimes have huge spaces between sentences (which is different from multiple spaces, by the way). Yes, layout software comes with preprogrammed find/replace queries designed to change all multiple spaces to single spaces but no such queries to carry out the reverse. Yes, your teacher in sixth grade probably did teach you to use two spaces and yes, that probably has at least something to do with the fact that she learned to type on a typewriter in a monospaced typeface. It's all true. It doesn't matter.
I type with single spaces because I think it looks and works better, and because I'm a designer who's been trained in the ways of typographic propriety. It physically hurts me to see two spaces after a sentence (or two consecutive hard returns, hyphens used in place of dashes, or a lowercase "fi" that doesn't form a proper ligature). It's an emotional, almost irrational need and a professional expectation, but there's nothing objectively, cosmically right about any of it. I don't have any statistical information at hand supporting the notion that single spacing produces a superior design aesthetic, although it seems like an unassailable fact to my eye, and is the kind of standard the powers that be in my field will hold me to unflinchingly. The same case could be made by a high school student whose English teacher demands two spaces (and probably 12pt Times New Roman, which...don't even get me started). That student should use two spaces, because that's what is useful and/or required in the circumstances.
So let's all take a deep breath and stop treating this as a black-and-white issue. When I get your double-spaced email I'm going to judge the hell out of you and your plebeian habit, and when you get my single-spaced email you can mentally dislodge the large, pointy stick from my butt (or drive it in deeper, if you prefer).