One, and one only! Particularly if you are sending a manuscript to a publisher for possible publication. Some publishers state that in their guidelines for a reason. With programs like Word, if you put in two spaces, the publisher will often have to manually remove them all when formatting for publication. How annoying. And you don't want to annoy your publisher!

I also had this problem, believe it or not. I was taught how to type in elementary school on ancient computers, and we were told to use two spaces after a period. I had to retrain my fingers before college, when professors got really picky out that kind of stuff. It was hard at first, but I did it!

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