I've actually been working on training myself to use one space, and it's a hard habit to change. I was taught to use two spaces by typing teachers, but was recently given a bit of a lesson from a friend who studies typography and designs his own fonts that the correct form is to use a single space.

Two spaces emerged as a convention for reasons mentioned in other posts here, and typing teachers in particular seem to have jumped on that bandwagon. However, the effect on rendered type is that it leaves unpleasant gaps in paragraphs in most popular programs (Word, etc.), and so the correct form according to typography professionals is apparently one space (or slightly more, but then that becomes a rendering issue, not one for the typist to decide). Since I have a huge appreciation for typography (albeit, not enough knowledge of the art itself), I feel it's worth correcting my habit.

I'm the opposite. I've used one space all my life but my AP English teacher wants us to use 2 spaces.

Go figure...

I came to write the same thing. I tend to use one space when posting in a forum like this but two spaces when I write a paper for class.