Two spaces after a period: Why you should never, ever do it. – By Farhad Manjoo – Slate Magazine.
Interesting… I’ve never been one to put two spaces after a full stop, but I know people who do. It’s always seemed like a generational thing to me, and I guess, given the typewriter origins of the double-space, that makes sense.
I know quite a a few people who have old school typewriters they use occasionally, but I’ve never noticed if they use a single or double space. I’ll have to take note next time I get something from a typewriter.
avocado and lemon 
I usually put two spaces after a period, but that’s the way I was taught. At that time, using a single space was doing it wrong. Of course, that was before computers were as common and most people still followed typewriter conventions. Once computers started catching on around, er 1995, either way was considered acceptable. Now apparently single-spacing is correct way and double-spacing is wrong. That’s the trouble grammatical ‘rules’–they constantly change to suit varying opinions.
What about for colons/semicolons? The way I was taught, it is double space after a colon, single space after a semicolon.
I’m not sure what the rule is there! I guess if you were on a typewriter and it made it easier to read if there was a double-space after a colon or a semi-colon, then you’d use it. But because I’m a single-spacer I never bother with either.
I’d love to look some more into the way words look on a page. It’s interesting how much it can effect the way someone reads.