Authorities Were Consulted, which resulted in helpful squibs from both Gordon van Gelder and Dr. Stan Schmidt concerning their opinions and experiences in the matter. Being an enterprising fellow myself, I sought (and obtained) the kind permission of both editors to quote them here on this blog as a public service for those of us who concern ourselves with the details of such issues.
Gordon van Gelder, when asked about his preferences, said:
"I don't fuss much over manuscript format and the question of one space vs. two spaces is something that doesn't cross my mind when I'm reading submissions, but I have to say that I do favor Courier and the manuscripts that I find the most readable are still the ones that follow the standard format (like this: http://www.sfwa.org/writing/vonda/vonda.htm)."
Dr. Schmidt's comment was:
Since I don't actually do the typesetting end of things myself, I recently asked Trevor the same question. He said he has no real preference between one or two spaces between sentences; we eventually use one, but he has a macro to change any doubles in manuscripts automatically. I also asked the production manager at Prometheus, for whom I'm currently writing a book, and her answer was similar: they prefer one, but consider it "too hard to enforce," and too easy to change, to try to hold anybody to it.
In my own writing, partly because of those answers, I've pretty much switched over to one space. There's another small reason, too: it saves a tiny amount (but it adds up) of time and wear and tear on your space bar (probably the most-used key on the board) and at least one digit.
As for font, I think most editors still have at least a slight leaning toward Courier, though at least in my case it's extremely mild; I'm generally content with either. I'm not sure I believe that "theoretically"; if anything, I think I personally find Courier a little easier on the eyes. But there's another reason why many editors prefer it: most of us at least occasionally find ourselves wondering about an author's word count and wanting to do an approximate one of our own, and that's easier with a nonproportional font.
While we're in this ballpark, let me add one other editorial preference that most editors seem to share and that sometimes surprises authors. Computers also make it easy to do actual italics instead of underlining, but most of us would rather you didn't, and continue underlining instead. Our computers can easily convert it in the final typesetting, but in many fonts underlining is much more obvious to the eye, and therefore less likely to be missed, than italics.
Finally, writer (and designer/typesetter) Stephen Stanley said this:
Double word spacing between sentences was a typewriter convention. The period key on a typewriter is one of the most often used. Along with the comma key. Imagine a worn comma key losing its "tail." Then imagine a poor typesetter trying to figure out if it was a period or a comma (sure, the next Capitalized word would be a clue, unless it was a proper noun -- and typesetters weren't required to read for context, it slowed them down). Typesetters, by the way, either set type by hand our re-keystroked the copy into a linotype machine. The extra word space after the period in a manuscript made it clear at a glance to the typesetter that it was indeed a period.
Proper typesetting has NEVER had two spaces after a period. Remember, manuscripts are TYPING not typesetting. Even now.
Now that layouts use copy supplied by the writer, to eliminate re-keystroking, it is unnecessary to use double spacing between sentences. (A simple global search/find eliminates the accidental multiple spaces that DO occur.) We have kept the convention during this transition phase because editors have been ambivalent. Apparently they are now loosening up.
If you use a proportional typeface (Times, etc.) instead of Courier there is NO WAY to roughly estimate the number of words by page count of a manuscript. The old 250 words per page no longer applies. So, if an editor does want to quickly confirm a writer's word count by the old way, it can't be done accurately. That's one reason I continue to use Courier.
The other reason is it makes a manuscript "neutral." The is no typestyling, no temptation to be typographically clever. I like the look of a neutral manuscript, but obviously it is the preferences of editors that should most concern a writer. Stan Schmidt has confirmed he doesn't have a strong opinion about Courier vs Times, except that he still wants a manuscript, not a story typeset for him to match Analog's layout (and I bet he gets them). Please take note about the use of italics. It disappears on a manuscript and is hard to find. As a graphic designer often rushing for a deadline, if I can't easily see the italics (i.e., underlined) I might not properly typestyle the italics. Ultimately the only person who will really care is the writer, so why set yourself up for disappointment? Underline italics on your manuscripts.