Two Spaces After a Period, by Mignon Fogarty, Grammar Girl Blog

http://www.quickanddirtytips.com/education/grammar/two-spaces-after-a-period

I learned to type on a typewriter. The rule was back then was to add 2 spaces after a period at the end of a sentence. When I first heard about the switch to 1 space rather than 2, I wondered why change?

The reason was clear. I was no longer using a typewriter, but a machine that automatically adjusts the spacing between characters and sentences. Makes sense to me. -CCE

Were you taught to put two spaces after a period at the end of a sentence? Many people were, but now most publications recommend using just one. Here’s the scoop.

If you learned to type on a typewriter, you’re going to hate what I say next: Do not put two spaces after a period. Don’t do it. Just use one.

I know. I was taught to use two spaces after a period in my high school typing class too, but you know what? It’s not that hard to break the habit. I haven’t been tempted to type two spaces for decades. It’s not like quitting smoking. I don’t find myself in nostalgic typewriting situations and suddenly get hit by an unexpected urge to type two spaces.

The modern and easy-to-follow style is to put one space after a period.

I’m not making this up to torment you. Typesetters write and beg me to tell people to only use one space. If you use two spaces, they have to delete them. Yes, it’s not that hard to do it with search-and-replace, but it’s not that hard to put dishes in the dishwasher either, and you don’t like doing that, do you?

If sympathy for typesetters doesn’t move you, I’m willing to bet you’re a rule follower. I don’t have a lot of to-heck-with-the-rules type of readers or listeners. And everyone who makes the rules today agrees: It’s a one-space world.

TheChicago Manual of Style, the US Government Printing Office Style Manual, ThePublication Manual of the American Psychological Association, and the AP Stylebook are just a few of the style guides that recommend one space after a period. . . .