I’ve recently seen a number of manuscripts and blogs with double spaces after terminal punctuation (periods, question marks and exclamation points at the end of a sentence). I remember learning to do that way in a very long ago typing class, and I’m sure that’s where these writers learned it as well. However, times have changed.
Now that computers offer proportional typefaces, rather than the fixed-width Courier, it is correct to space only once after a sentence. Double spaces make unattractive gaps or “rivers of white space” down the page, and prevent the manuscript from looking professionally typeset. It may be a hard habit to break, but it’s worth it.
If you’re still a little dubious, or if you have a typing manual or teacher who still admonishes you to space twice after terminal punctuation, you may want to refer to the ultimate authority, The Chicago Manual of Style. The recently published 16th edition addresses “period/single space after” in sections 2.9, 2.11, 6.7. If the CMOS decrees it, you can be assured that it’s so! If you don’t have this edition on your shelf, you can sign up for a free 30-day trial online and see it for yourself.