How many spaces after a full stop?
This is one of the issues that is still argued over (online and in real life), even though the style guides all agree that you should use (place your bets now) … only one space after a full stop (or period, for American readers).
Many people, me included, were taught that the correct way to format was to use two spaces after the end of a sentence (sometimes called English spacing). Most of these people still think this is the correct way to do it.
Typography professionals and style guides disagree, however. Many websites will automatically remove superfluous spaces as well. The reasoning is that the double space is no longer needed. Originally, it came from manual typesetting, where all letters were the same width (monospaced), and it was thought that using double spaces after full stops would help the reader see the spaces between sentences. These days, with proportional fonts (where, say, the letter M is wider than the letter I), it is argued that double spacing is ugly and obsolete.
This post on Slate.com slams double spacers, but, to be honest, if you aren’t a professional, does it really matter? Especially with so many ‘laypeople’ thinking that two spaces are acceptable, if not correct, it makes vehement single spacers seem a bit obsessive.
Further information: Wikipedia.