Single space. I learned double spacing in high school on a typewriter, but have gladly switch to single spacing.
APA style says to use double space after a sentence.
AP style says single space.
I did both in college – I used single space if I was doing any news writing (because space on a newspaper is a premium) but I would double space anywhere else. Double space just looks right to me and I am happily passing it on to my kids.
You’re doing everything right! 
Seriously, the two-space thing is something I could hardly care less about, but by force of long habit I do tend to two-space even when writing in Word with proportionally spaced fonts. You really do need to do it with monospaced fonts and I find it easier just to be consistent.
Or you can use a formatting tag like “ ”. HTML also doesn’t let you insert multiple blank lines, which it collapses into one. So what? Some geek’s arbitrary decision on HTML standards is hardly the authoritative last word on optimum typography. And that HTML behavior is often a PITA. The fact that typographical practice is to use one space with typical typographical proportional fonts is a lot more persuasive. But it’s a trivial matter of style, like whether paragraphs should be denoted by a blank line or by an indent (or, heaven help us, by both!). Or how and when to hyphenate words across line breaks.
That actually wasn’t my point. I was wondering how many people realize that their two spaces are being turned into one, anyway, and how many even notice.
Two spaces after the period and colon.
One space after comma or semi-colon.
Spaces around en dash or em dash, but not around hyphens.
That is not what my style guide says, although it’s the Fourth Edition from the 90s. My rule 4.11 says:
Reading farther down, I do not see anything about spacing twice after any punctuation, just about not spacing after periods in an abbreviation.’
ETA: Actually, the fact that I’m using the Fourth Edition is the rub. The sixth edition changed it to two spaces. All other editions had one. I’m surprised. I would have guessed, if anything, earlier editions advocated for two and then switched to one, not the other way around.
And, of course, the decision by the APA to go to two spaces (at least in draft manuscripts) was met with a good bit of controversy (read the comments.)
Two spaces 'till I die, baby!
And I like my commas, too.
One space. Join the 21st century (or even the late 20th) people!
Do you all still double pump the gas pedal before you start your cars? Embrace change.
As a former professional writer, I always use a single space because the Associated Press and Chicago Manual of Style both said so (at least in 2003, when I freelanced for a living).
Now that I’m a product manager, anytime someone asks me to look at their work, I compulsively reduce the double spaces between sentence to one if they’re there.
What does this even mean?
Spaces are cheap, I always use 2 between sentences.
Bolding mine.
So is the issue that “somebody” notices, which implies a small number of people, maybe even one, or that “EVERY person” notices, which implies, well, every single person? There seems to be a bit of a disconnect here.
Again, I’ll say I don’t notice, so I certainly don’t consider the extra white space to be “valuable” in any way. Speaking only for myself, I would not be one of those people who would call up and complain. (Even if I noticed, I wouldn’t be one of those people because I don’t care one way or the other how many spaces someone else uses, given that, you know, I don’t notice.)
Just that if your argument is based on some kind of universal, that EVERYBODY benefits if we all would just use two spaces after a period, I’m here to tell you that you’re wrong :).
Double space after punctuation.
J Easterbrook of the 7th Cir, has major issues with that. Claims the white space is distracting. I disagree.
I’m a freelancer who gets paid by the word. Ain’t no money in extra spaces.
All the books, magazines, and newspapers you’ve read in your entire life have looked odd to you?
APA style is for submissions typed on a typewriter in monospaced font. Those double spaces don’t show up in the publication.
If you’re using monospaced fonts everywhere, then double space. If you’re using normal proportional fonts, then no, double space does not look right to you.
It does say “manuscript draft” in the Sixth Edition (or at least it says so on the APA’s blog post about the change, although a commenter states that no such distinction is made in the text of the Sixth Edition itself), but is it standard to submit submissions drafted out on a typewriter or in Courier (or similar monospaced font)? I actually don’t know, but that’s why I was surprised that this is something that crept into the sixth edition, published 2009. I could understand it being there in older editions, where draft manuscripts were routinely typed, but these days?
I’m a double spacer and am both willing to argue the point, and have argued the point. (I’ve written a book, and asked a friend to proofread it, and she promptly took out all the double-spaces. I promptly fired her.)
And no, the supposed arguments in favor of single-spacing being better are all bullcrap, and obviously so, as can be determined by casually examining the fonts in question. They are literally founded on lies. Single-space may be ‘correct’ due to fiat, but it’s certainly got nothing to do with how the actual fonts look.
I honestly don’t think it really makes much of a difference either way, and I’m willing to bet the double spacers don’t notice that most of what they read does not have two spaces after a sentence terminating period. Similarly, unless the spaces are exaggerated, I doubt that most people notice when there’s two spaces instead of one at the end of a sentence. (Although I have seen some examples where it drives me absolutely batty.) The thing is, the period already carries so much space with it, that a period followed by a space already looks like two spaces and visually breaks up the line quite fine. I think it’s aesthetically more pleasing with a single word space or a sentence space (which is slightly bigger than a word space, but not as big as two spaces. Maybe 1 1/2 word spaces.) And type setters seem to agree (feel free to read this Wikipedia article which is pretty informative and neutral.) That said, it may be because that’s what they’re used to. But, then again, I’m a stickler about not using hyphens when an en dash or em dash is appropriate. (For example, in denoting ranges, I always use an en dash, not a simple hyphen.)