During the past year, three different authors have submitted stories to us in which there were two spaces after every word, which made the stories very hard to read.
It took me a while to figure out what had gone wrong. And I wasn't going to post about it when it was just one author. But after seeing three authors do it (two of them in the past three months), I figured it was worth making a public comment. This entry is not so much aimed at my regular readers; more at people who are doing web searches looking for manuscript formatting info.
When I was first beginning to submit stories, I read various manuscript-formatting guidelines, and they all said manuscripts should be "double-spaced." And I wasn't sure what exactly that meant.
But eventually I saw examples of proper manuscript format and figured out that in almost all publishing-related contexts, "double spacing" refers to blank lines between lines of text, not to extra spaces between words.
To put it more directly: No magazine ever wants to see two spaces after every word in a story.
More generally, if you aren't sure what standard manuscript format (SMF) looks like, Google for standard manuscript format and read some of the resulting pages. They don't all agree on all the details--for example, font choice is a particularly contentious subject--but they all provide useful info and most provide good examples. If you've never seen an example of SMF, then look at an example or two before you try to submit your work anywhere.
Btw, an author asked us a few months ago whether stories to be submitted to Strange Horizons should be double-spaced or single-spaced. The answer is that, as long as you use our web form to submit an RTF-formatted story, the line spacing in your story doesn't matter to us; our conversion system changes the spacing between lines to what we prefer, so you don't have to worry about it.
But our system doesn't change the spacing between words. So please don't put two spaces between words.