One space between sentences! That’s right! I wrote a long blog post about that several months ago announcing this and explaining why. There had been a lot of confusion about this and I kept getting asked about it. Basically
when computers came on the scene, only one space was necessary in text for clear reading. So that's what publishers decided; before that, with typewriters, it was two spaces.
But since then, I have discovered that there are exceptions. Lawyers use two spaces in legal briefs that are read by judges and others. Why? I guess it's for clarity’s sake. I admit it took me a lot of sleuthing to find this out. I even got hold of that Bible of the legal field—the Blue Book—which explains how to document all references.
And what did I find? The narrative in the book, typeset through computers, had sentences separated by one space. But when the Blue Book gave examples of actual legal briefs, the text clearly contained two spaces between sentences. My impression is that this is for clarity's sake--for the judges and other readers. So…I find myself corrected, or, let’s say I am informing you (if you are a lawyer) that of course you need to stick to the conventions of your field. For legal briefs, it's still two spaces between sentences.
Good writing skills are a cornerstone of success in the academic world, and they are also a major plus in later life.