How Many Spaces After a Period?
Back when typesetters needed the extra space to show where the sentence ended, a double space was necessary. Typewriters later used monospaced letters which meant letters took up the same amount of space--so thin characters would get the same space as thicker characters and it would be hard to see where the sentence ended.
Later, and still today, people continue to use double space as a rule they think is still in effect, or maybe they use it out of preference. I had started using double space myself a while back because I thought it was the right thing to do. A friend had finally convinced me that it was correct and I did a bit more research and found that a lot of people still did use it. So I jumped in and before long I became used to it.
Years later and I'm reading how wrong it is and how everywhere people aren't using the double spaced rule any longer. Agents and editors are more often even accepting different fonts with submissions, as well. Computer word processing programs have fixed the problem of monospacing and it is easier to see where a sentence ends now, making it unnecessary to double space.
Courier is still preferred by a lot of editors because all the characters line up equally for a word count of 250 words per page, it can be done so without the aid of a computer--keep in mind that would be a page with 1 inch margins and double spaced sentences. Electronic and disk submissions diminish the need to count words manually.
Getting back to the double period, writing manuals say always use a single space. Professionals on writing websites and others say single space. Do a search with the key words (should I double space) and you'll get tons of info on the subject.
People who grew used to the double space continue to use it, and that's ok, but if you're a new writer starting out you might want to use single space to not give agents/editors one more reason to toss your manuscript in the slush pile. I read a tip somewhere that you can still use double space if you're used to it, but later on go to the replace option on Word (if that's what you use -- or whatever similar word processing program you use for your OS of choice) and on Find What: you press space twice, and Replace With: you press space once. POW, you have the document formatted with single spaces after periods and ready to go.
Until recently, I couldn't find a definitive answer to this one space rule. I had fought internally with it for years--OCD--and finally came to the conclusion that using single space is the correct way to go, though sometimes I do hit that space button twice.
Now it's everywhere, like the writers are looking at you going, "What? You still use double space?" And then they turn up their nose and scoff with a nostril huff. C'mon, you should have known, sheesh.