I was searching for info on this particular topic, and stumbled across this page. Personally, I was taught the period-space-space rule as an early teenager, at a time when very few people had computers. If you were lucky enough to have one, your family was rich, or it was a business computer. As computers became more mainstream and typewriters were phased out, the standard shifted to simply using period-space. I was told at the time by someone that I had no reason to doubt, a highly-educated teacher, that the period-space-space rule had been changed because it simply looked better to have a single space on anything you view either on-screen or on paper. The need for it back in the days of typewriters was always attributed to the fact that in many cases, the size of the "space" itself on a typewriter was just too small to make a document very readable without the double space.

Many years passed since I was told that the period-space-space rule was replaced with a period-space rule, and I never gave any of it much thought until it turned into a heated debate at my job recently. It seems that the advocates of the single space between sentences are on my side of the digital divide, and those that absolutely NEED to have the two spaces reside on the other side of it. And, just like this period-space-space debate, there are many different and valid definitions of the digital divide, so let me define it as I'm using it here:

Digital Divide (for this particular use) - I'm referring to the gap in technological skill between the older generation and the younger generation. Certainly there are several divides of this type across generations, but for my uses, I'm talking about a split somewhere around the age of 35 or so. Basically, anyone under 35 had some (perhaps limited) access to a computer during their childhood. Anyone older than that, well, computers just weren't around that long -- at least not personal computers available to the average user.

So, as I said, the debate on this in my office (and in various other places on the Internet) seems to center around those who typed papers for school on a typewriter, and those who did it on a word processor. But in reality, what's the difference? Seems to be just that you practice what you are taught.

As far as readability, well... I think that's mostly opinion. Go ahead and do study after study on it. It's still just an opinion. One of the comments above says that they've done study after study after study on it:

"Depends - if you want people to read faster, more easily, and with better comprehension, use two. The data on this is solid, overwhelming, and has remained consistent on studies up to and including the latest brain-imaging technology. Even people who are young enough to have very little experience with the double-space after period convention read better and are more likely to read text when it is used."

OK, that's all well and good. But I completely and totally disagree. My eye catches EVERYTHING. So if there's a misspelling, I see it. If there's a grammatical error, I see it. If there's an extra space after a period in just ONE sentence of an entire document, I see it. If there's an extra space after EVERY period, well, frankly, that just drives me nuts. It causes too many rivers. As I read, my eyes move left to right much more skillfully than their ability to stay on the same horizontal plane. So if I'm reading and there's an extra space after a period, there's a good chance that my eyes will get pulled with the extra space to the line above or below. This results in much poorer reading comprehension, and certainly adds to the time it takes me to read something. I don't believe I am a completely unique case, however, certainly I understand that there are people out there that don't ever have a problem like this.

Nonetheless, I submit to those of you that say, "ALWAYS USE TWO SPACES," -- I believe if you're going to have a blanket rule, it should err on the side of always using ONE space. If you use one space with a monospace type font, it results in a very nice, easily-readable document, despite the fact that it may not be the standard. If you use two spaces, it results in lots of holes and white space where it shouldn't be, even if it's the way it's SUPPOSED to look. On the proportional-type side of the coin, if you enter one space, it is proportinally sized as it should be. If you enter two, the proportionality goes out the window and it just looks wrong.

I tried very hard to give examples of this right here, inline with the comment, however, it seems HTML is disabled, or at least, it's disabled through the preview of the comment. For that reason I couldn't include the examples I wanted to include.

If you want to see the differences, copy the paragraph below and paste it into Notepad. Save it as "whateveryouwant.html" -- then open it in your browser. You'll see those big, glaring, extra white spaces in there, and it's not even about readability. It's about the fact that you'll pause before you even start reading -- because your eye is drawn to those big, ugly white spaces.

[copy below this line]
Depends - if you want people to read faster, more easily, and with better comprehension, use two.  The data on this is solid, overwhelming, and has remained consistent on studies up to and including the latest brain-imaging technology.  Even people who are young enough to have very little experience with the double-space after period convention read better and are more likely to read text when it is used.  The paste-up and typography issues are a red-herring.  The reason was always to make it easy to skim to the end of a sentence.  Most good readers locate the end of a sentence, before reading the whole thing.  When there is no easy way to peg the end of the sentence, reader may either ignore many of the words in the sentence, or else they read every single word all the way through to find the period, then having identified the sentence, they go back and read the sentence as a chunk.[end copy]

Now, to see what that looks like in a monospace font, copy that same paragraph as it's displayed in your BROWSER (after you save it as HTML, not from Notepad) and paste into Microsoft Word. Now change the font to a monospace font -- I use Courier New but use what you like, just make sure it's monospace. Wow, look at that, holy-white-space batman! It's even worse in monospace.

So the conventional wisdom is that if you're talking about a monospace font, use two spaces, but if you're talking about a proportional font, use one. I submit to you the above example. In my opinion, neither of them look good with the extra space. But if I had to choose one to have the extra space -- it would be the PROPORTIONAL font! Crazy. Anyway, with all of this research in my mind, I'm going to stand firm and say that I will NEVER use period-space-space, and my advice to anyone that wants to would be to go through an exercise like the above.

If you want to see what kind of change it might make in a document you already have, it's fairly easy to do. Just use the Find-Replace command. If you're a period-space-spacer, just do a Find on ". " (that would be period-space-space, and don't use the quotes) and replace with ". " (period-space). Now, I don't know exactly what that will render as when I submit this, because HTML kills extra spaces, and if I hard-code a space on this blog, it renders as   without converting to what it actually should look like. But the point is, when you're using the Find-Replace command, in the find field you should type a period and two spaces (if you're a period-space-spacer) and Replace with simply period-space. Then decide for yourself which version you like better. My vote says period-space, and I've illustrated why above. But my decision has nothing to do with your personal likes or dislikes, so decide for yourself.