Why you should never, ever use two spaces after a period.

The OP says that double-spacing is an obsolete holdover from the typewriter era, where the extra space made monospaced type easier to read. I'll go one further and say that spacing sentences using space characters -- any number of them -- is obsolete.

In the present era, the act of typing is separate from the act of typesetting. The comment I'm typing now may be typeset in Arial in Chrome, typeset in Ubuntu Mono in Emacs, or read aloud by a software program to a blind person. No prescribed number of space characters is going to be appropriate for all cases.

The reading software, not I, should be responsible for locating my sentence breaks and setting appropriate spacing there. Perhaps in the future we'll assist the software by marking up sentence breaks using a special character sequence. Ironically, a double-space would serve that function pretty well.

"Typing two spaces after a period is totally, completely, utterly, and inarguably wrong."

The funny thing is, when I read this article, one space after a period looks wrong to me.

"Everyone—everyone!—said it was proper to use two spaces."

Language and usage evolve. If everyone thinks it's proper, eventually it will become the norm. Even if you personally feel it is wrong. In this case, Most people have been taught to use two spaces after a period.

"Typographers, that's who. The people who study and design the typewritten word decided long ago that we should use one space, not two, between sentences."

Alexander Graham Bell originally wanted us to use "Ahoy" when answering the phone. It doesn't make it the standard or wrong if society decides use something different.

I must admit, that I am guilty of being a double-spacer. Your argument and sources however seem to be sound, so I think I shall give a go at doing it the correct way from now on. I've been becoming more and more interested in typography lately, so let's bring this movement to the forefront and starting single spacing after punctuation again!

(Notice: The double-space has been so ingrained in my typing for so many years, I even did it in this comment without even realizing it!)

Here's an interesting rebuttal. http://www.heracliteanriver.com/?p=324

I think two spaces are okay. I use two spaces in source code comments. Source code is monospaced after all, so the "kerning excuse" doesn't apply.

FWIW, the default Emacs sentence delimiter is a period and two spaces.