Can I let you in on a secret? Typing two spaces after a period is totally, completely, utterly, and inarguably wrong. [...] And yet people who use two spaces are everywhere, their ugly error crossing every social boundary of class, education, and taste.
As a US Marine, I know that what’s right is right and you are wrong [about the spacing issue]. I declare it once and for all aesthetically more appealing to have two spaces after a period. If you refuse to alter your bullheadedness, I will petition the commandant to allow me to take one Marine detail to…impose my rule. Thou shalt place two spaces after a period. Period. Semper Fidelis.–from the CMOS forum
6.7 Punctuation and space—one space or two?
In typeset matter, one space, not two, should be used between two sentences—whether the first ends in a period, a question mark, an exclamation point, or a closing quotation mark or parenthesis.