I’ve been (or should I say HAD been) following a LINKED IN thread where the query was made, “one space or two” after the end of a sentence. Over 640 comments have been made, over and over, beating the dead horse.
Fact: anyone who was taught on a typewriter, was taught to put in an additional space at the end of the sentence because the font was fixed, ie the “i” took as much space as an “m.” So, to visually break up the sentences, you added an additional space.
With the advent of word processing and proportional fonts, there is no need for the additional space. Visually it is more appealing to use one space; additionally, the way your copy wraps or is set in a brochure, for instance, can change dramatically with the addition of extra spaces.
For some reason people keep arguing about this. Now, I know this isn’t creationism vs. evolution, but to someone in the business of communication, it is relevant.
Here’s a great article on the subject:
Two spaces after a period: Why you should never, ever do it. – Slate Magazine.