I always put two spaces after a concluding period! Perhaps a bit archaic, but it used to be the standard (and is still "acceptable" according to the [Modern Language Association's FAQ about it](http://www.mla.org/style/style_faq/mlastyle_spaces)).  [This Slate article](http://www.slate.com/articles/technology/technology/2011/01/space_invaders.html)'s author is pretty adamant about a single space, but then in one paragraph states that two spaces "was" used reasonably with monospaced fonts (as if monospaced fonts aren't used any more and "went out in the 1970s!").  Oh, great, and now I see that when github renders markdown, it removes double-spaces after periods!  Unbelievable. ![](http://www.reactiongifs.com/r/wwyp.gif)