Ok, so I’m 46 years old and when I was young we had rotary telephones, vinyl records, rode on the backs of dinosaurs, etc. Therefore I wasn’t born knowing that we only put one space–not two–between the end of one sentence and the beginning of another. In fact, I will admit that, like most people of my . . . er . . . vintage, I was suspicious when I started to hearing or reading, I can’t remember which, that I was doing it all wrong.
It turns out that the single space rule is older than I am. According to the oracle (i.e., Wikipedia), the last known official US government document to specifically prescribe double spaces after concluding punctuation was a 1959 government style guide.
Lest you doubt or hate, the 2000 and 2008 editions of the Government Printing Office’s (GPO) Style Manual are unequivocal in their guidance regarding this convention: “A single justified word space will be used between sentences. This applies to all types of composition.” Modern Language Formatting Style (MLA) General Guidelines state: “Leave only one space after periods or other punctuation marks (unless otherwise instructed by your instructor).” There’s apparently also something called the Associated Press Style Book which is equally clear on the subject. And the Chicago Manual of Style. And Typography For Lawyers.
I’ll admit I’m struggling to overcome the sheer habit of double-spacing. I had to go back and fix several sentences in this post.
-Thanks to Dmitriy Kopelevich for setting me straight and providing the background materials.