In writing, that is. Ha ha.
I was taught in typing class way back in the day that you ALWAYS double-space after a period. This is just the way typing works. More and more I’ve noticed that not happening so much, though, whether by a forced removal of the double-space by a program or people just not bothering with two spaces after periods when they write.
The practice of double spacing after a period is a holdover from the days when typewriters had “monofaced” type, meaning fonts in which every letter had exactly the same width (in other words, “m” used the same amount of space as “i”). Because of the monoface font, two spaces after a period made it easier to see where one sentence ended and another began.
Today, however, most fonts we typically use are proportionally spaced, meaning that characters take up an amount of space relative to their actual width (the “i” uses less space than the “m”), so that double spacing after periods is not only unnecessary, it mars the look of your text by scattering it with small gaps. Books, magazines, brochures, newspaper, desktop publishing, etc., use only one space after a period, as do both AP style and The Chicago Manual of Style.
Personally I’ve never found the double-spacing to be an issue unless it’s forced words to another line when I don’t want them to be (typically in webdesign and the like). Overall, the gap doesn’t seem awkward to me or that it mars the look but it seems natural, like there’s supposed to be that extra gap, because that’s the way you’re supposed to type.
I just can’t break myself of the habit, though. I’m sure that if I tried and worked on it that I could be rid of it over time, but why bother? Is it so bothersome to the reader that I must change what has been the industry standard since typing began? Perhaps I should just pick fonts that need the double-space so there’s no awkward spaces and the like. Or perhaps people should just not be so damn picky.