I need help settling this disagreement with a friend: One space or two after a period?

These days, you only need one space after a period at the end of a sentence. The confusion arises, however, because the rule used to be two. Why the change? Technology, of course. On most typewriters, every character took up the same amount of space. The letter “l” had the same width as the letter “w.” The same was true for punctuation. So, two spaces after a period created a much needed visual break. On computers, however, characters are proportional. That letter “l” is narrower than the letter “w.” So, an extra space at the end of the sentence doesn’t help make the text any more readable. Most style manuals agree on this point, but some state that either choice is right. Still, for your manuscripts, it’s best to stick to the convention for published work, which is one space.