I'm wonder if the use of making two spaces after a punctuation mark at the end of a sentence make any sort of difference when trying to get published in a magazine.
I agree with wordwalker. Use of two spaces is archaic at this point, in my view. I preferred to get manuscripts with only one, though I didn't ask for it in guidelines. I don't know if anyone addresses this specifically in guidelines anymore, but with computers and word processing, I'm not sure there's a good reason to use two anymore.
I think the need of it was from mono-space type, when the end of a sentence had to be clear[er]. With modern type setting and proportional fonts I guess it is less important. In fact I only know of one place that used to require it, an EU technical journal.
As has been said above, two spaces has been largely phased out. If you do use two spaces by habit, it's easy enough to find and replace two spaces with one space.
I don't think it will stop a publisher from accepting your manuscript if they like the story. It's an easy enough thing for them to change when they format for publication. But it is something they're likely to take out, so it's worth training yourself not to do it, for the long run.
I personally have used two spaces simply because that's how I was taught (I learned on a typewriter originally, showing my age a bit) and it's hard to break the habit. However, I'm aware that some publishers have specific guidelines they want followed, so I rather just go back and fix it then potentially have a problem with my manuscript due to formatting issues.
I wonder how two spaces will work on an e-reader if the font settings are large. I know a lot of people who turn the font size up pretty high on Kindles and Kobo Readers, etc., and I wonder if two spaces won't look over-emphasized when people do that.
I recently had a little debate with a colleague over the double space. I learned in school that when handwriting you leave a little extra space to make it clear the sentence has finished and make it easier for the teacher to read. I still do that. But school never taught typing - neither primary nor secondary. Primary school only had four computers in the whole building, including the headmaster's, and secondary school IT was about using Word and Excel for homework purposes, at an alarmingly slow pace. I learned typing from typing up stories I wrote and from Mavis Beacon Teaches Typing CD, which I didn't much like so never got far with, but mostly that was about typing four letter words faster than ants ate the letters, and either didn't go into sentences or I didn't get far enough with it. I don't recall being taught either way; but on the net (when we finally got it, buzzing and growing dial-up) I mostly read single spaced, so that's what I wrote.
Anyway, point of my little debate with my colleague was that he's old. I don't recall the friendly insults he slung my way, something about being obviously wrong, young whippersnapper upstart, something like that (he's about 7 years older than me). In the end we agreed to disagree and he started talking about swearing in the workplace and the site office with the other guy in the room at the time.
Double spacing after a period is one of those idiosyncratic things that will make me notice the writing, if the space is big enough. I get document from clients sometimes, with double or even triple spacing, and depending on how the page is justified it can really look weird and always catches my attention.
That's why I asked the ebook question, because you have to remember people are using different screen sizes and readers can customize their font size as well. If a double space will end up looking like a lot of white space on a given e-readers, it's going to look weird and make the reader notice the formatting. That's a good reason not to use double spacing, in my view.
When I was first taught typing I learned with a double space, but I trained myself out of it (which actually isn't as hard as you might think) after being told by an editor (of an academic journal) that it wasn't preferred anymore.
I agree that it would look odd in an ebook; and the last thing you want a reader thinking about is formatting, because then they're not thinking about the story.
I was taught doublespacing, and I still do it that way. Online if you doublespace, it only shows up as a single space, so I think that's trained people out of reading it.
It's easy enough to do a search and replace, and I've actually run it on my documents to go both ways. Consistent formatting is more important than anything else.
I have noticed that double spacing can look awful as the font size goes up or if you justify your text in small columns or without hyphenating. But I wouldn't have thought many people turned up their ebook font sizes so much for it to be an issue - that surprises me to hear.
It seems pretty clear that the going opinion is that doublespacing is obsolete, but I can't imagine anyone is going to care if you doublespace in a manuscript. In most work settings you're still going to have people who learned either way, so I'd call it mostly a non-issue until you get to the final print or ebook layouts.