I always think of 1990’s health guru Susan Powter when I see someone using two spaces after a hard stop (period, exclamation point, question mark, etc.) in 2009. Not only is the reasoning behind the double space antiquated, but it drives copy editors crazy! As Ms. Powter would say, “Stop the insanity!”
It’s 2009, not 1989!
The two-space-after-a-hard-stop guideline started as a way to make typing on a typewriter (remember those?) more legible since the amount of space taken by all keys on a typewriter is uniform. For instance, an “i” takes up as much space as ‘w.’ As a result, an extra space was added after a hard stop, believing that it would make the print easier to read since breaks would be more identifiable.
We could argue that point, but it’s moot anyway. One of the few places you can still find a typewriter is in the landfill.
Extra Spaces Deleted
Many online software content management systems will automatically delete any extra spaces inserted into a text file, including those after a hard stop. It’s a waste of time to put them in.
Nearly all Web style guides state that there should be a single space after a hard stop, so if they’re not automatically deleted, they’ll be manually deleted by an annoyed copy editor.
Publications (books, newspapers (destined to go the way of the typewriter, perhaps?), magazines, and brochures) are nearly universally printed with one space after a hard stop. They’ve moved on–can’t you?
Awkward Text
Word processing software packages often automatically justify text (which adds and subtracts space between letters throughout the entire line to make the right margin consistent), or at least include that option. An extra space in this format looks especially awkward.
Typewriter-to-Person Communication
The ony type of communication that the double-space is considered acceptable is any type of typewritten material that is not also printed in an electronic format, so unless your office hasn’t “upgraded” from the file cabinet to a computer, it’s time to “upgrade” your punctuation.
–Paul
Everything Language and Grammar 
I had a very hard time breaking myself of the habit that was forced upon me in a junior high typing class.
Yes, I understand.
In fact, that reason–it’s a habit–is probably the main reason that most people who use the double space continue to do so. I’m sympathetic–just not as much as I was before I started to copy edit a Web site! lol
Oh my. I never received the double space memo either. And I realize, as I am typing this, how completely ingrained a habit it is.
Thank you for giving this its due attention.
Oh my, I had no idea that my double spaces after my hard stops were driving anyone crazy. And some of my best friends are editors. Why haven’t they told me? I have been doing that my whole life. I don’t think I can break the habit. There, I did it, but it took a lot of thought. (See I can do it) Why doesn’t “Microsoft word” correct me?
Oh, I know one of your best friends———and it drives her crazy!
Thanks for the pictures for the blog (Autumn in Central Pennsylvania), by the way.
–Paul