Once, a long long time ago, in this here galaxy in which we live, you wrote on a typewriter, and the rule was two spaces after a period. And while even the nefarious Writer’s Digest acknowledges that no, we do not do this anymore, people insist on it, and give many a reason for it.
When we used typewriters, readability insisted that two spaces come after every period. It gave the eye a rest, and the irregularities inherent in typewriter text made it necessary.
We do not do this in today’s computer-driven world. I understand this is a hard habit to break – that’s what “Find/replace” is for. To remove all those double spaces from your finished manuscript before you send it in. Why, you ask?
Publishers don’t want to see it. There are plenty of other reasons, but the fact is that your manuscript needs to be clean when sent to an agent or publisher. If you’ve double spaced after every period, or tabbed to indent at the start of every paragraph, you’re not going to make it through the initial review process. Yes, some of them will fix it for you, but it won’t make for great relations. Considering all the other stupid things they’re going to want to change about your book, wouldn’t it be easier to just take out those double spaces?
I am speaking from experience. It would.