In case you’re not up to date on the current American Psychological Association guidelines, they decided in their recent let’s-hit-the-college-kids-up-for-more-money new edition of their style guide (Don’t buy it, by the way. Go here instead.) that we now have to put two instead of one space after each period.
That’s why this email I got from Reanna is funny:
The two spaces after a period convention came about in reaction to the monospace fonts of typewriters (they’re so gappy that the extra space helped to differentiate the end of the sentence). Now all the fonts we use are proportional (except courier), and typesetters agree that two spaces is wrong.
“We would never accept teachers pushing other outmoded ideas on kids because that’s what was popular back when they were in school. The same should go for typing. So, kids, if your teachers force you to use two spaces, send them a link to this article. Use this as your subject line: “If you type two spaces after a period, you’re doing it wrong.”
Somebody tell the APA.
Nathen's Miraculous Escape 
January 24, 2011 at 9:35 pm
Actually, from the fine article:
But yes. Computers can adjust the space between characters for legibility — a process called kerning — and so one space is plenty in any system that can set type decently. If not, it’s the kerning that should be adjusted, not the number of spaces.
I really wish for a typeface that kerns a single space properly, but treats two spaces in a row as an inch long gap. That should fix things once and for all.
January 28, 2011 at 10:35 pm
Hmm. I’m not sure where they got that from. My 6th edition APA manual says only “Space twice after punctuation marks at the end of a sentence.” I can’t find any “allowing” two spaces or “recommending”one. Perhaps there is a separate APA guide for publishers that says that. Meanwhile, college students across the land are losing points for forgetting to double space after periods.