One of the many ways I, as a freelance writer, cobble together enough income each month to pay the electric bill is by taking on the occasional editing job. Right now I'm editing a Chinese-restaurant guidebook, and last month I was working on a couple of magazine pieces.
One thing I see time and again is the persistent use of two spaces after a period or other final punctuation. This relic of the typewriter era has had incredible staying power. Even many young Millennials, who have never used a typewriter for anything, have inherited this habit from their parents or schoolteachers, or from following the examples of others.
Yet if you look at modern, professionally published material -- such as books from HarperCollins, magazines, newspapers and professionally produced websites -- you will not see two spaces after a period. In addition, most web browsers automatically render one space after a period no matter how many spaces appear in the source material.
There are a number of reasons why two spaces after a period are no longer necessary, but the main one is that the typefaces we use today (Times New Roman, etc.) are proportionally spaced. On a typewriter, the typeface (Courier) was not proportionally spaced -- every letter and character used exactly the same width. But with proportional spacing, the "i" is narrower than the "m" and -- most relevant to the current rant -- the space is already the right size for a visually pleasing break after the end of a sentence. Indeed, if you use two spaces after a period with a proportionally spaced typeface you risk creating a visually unpleasant quantity of white space.
I hope you'll all join me in eradicating the two-spaces tradition once and for all. Or, if not, I'd love to hear why.
Chicago Manual of Style Q&As on this subject