The vast majority of professionally printed material is typeset (typeset, not typewritten) with one space, not two, following periods and other ending punctuation. But that doesn't mean everyone thinks it should be that way as shown in these reader responses to the question of one space or two -- what do you do? But how about text we read on-screen?
Recently Jennifer Krynin tackled the topic of spacing after punctuation on Web pages for About.com Web Design/HTML and Web designers (Does anyone still use two spaces after a period?)
Dwight B. posted this comment to her blog, which is representative of many of the 2 space proponents:
"Yes, I still use 2 spaces. I always use some extra NBSP to get the effect. I learned to do this in typing class and still prefer the visual break, even in proportional fonts. I also try to make use of M-dash and N-dash to get different looks. I get amused over how much web-designers will kill to get rounded corners and let the text go willy-nilly. They'll except badly wrapped text as long as they get gradients and drop shadows."
I'm firmly in the one space camp on this issue (print or web). And yes, I'm old enough to have learned to type on a typewriter with the 2 spaces practice drilled into me by my typing teacher. But, I also read a lot of books and rarely ever encountered those 2 spaces. To me "willy-nilly" text would be text that sticks in those extra spaces (or text that was inconsistent in the use of spaces). Unnecessary and ugly, to me.
Now I know that many people feel the extra space aids readability. Maybe it does, for them. For me it hinders readability. It's distracting. For a one-spacer, the fact that HTML strips out extra spaces is a time-saver. Unless of course you take that extra step of putting in non-breaking spaces. Do you? Do you see the one space or two issue differently for print and Web? If you prefer 2 spaces, do you make sure to use them on Web pages as well as in print?
Read About It:
- Use One Space Between Sentences (About.com Desktop Publishing)
- One Space After a Period (About.com Web Design/HTML)
I’m with you in the 1-space camp & Itoo am old enough to have learned the 2-space version in my typing class so many years ago! In fact it was so many years ago we only had 2 electric typewriters in the class, so we would take turns using them.
Working for a newspaper I learned to use just 1 space, all those saved spaces means you can squeeze more type in a smaller amount of space. At least that was the theory that our paper went on.
My newspaper days also left me not checking my spelling
that should be “I too” not “Itoo” I’m used to a proofreader!
I, too, learned to type on a typewriter – a big iron underwood! But when I edit my local church magazine, I leave one space after a full stop (period in the US), as I was taught by the previous editor, who had worked for a major publishing house. So I suppose you can teach an old dog like me new tricks…
Using 2 spaces after a period is archaic. Typesetting on a computer does not produce equal spaces for every character as a typewriter did. Leaving 2 spaces can wreak havoc on the natural text flow of the font used. Word processors and page layout programs have controlled automatic tracking. Because of this, a double space used in one area may not be the same size as a double space used elsewhere in the very same paragraph. This can cause odd looking text flows. I’ve worked for a printing company for over 15 years doing various aspects of desktop publishing. One of the first things I often end up doing is changing a client’s double spacing to single so that the look of a body of text doesn’t produce weird patterns and odd blank areas. And yes, I am old enough to have learnt to type on a manual typewriter, but not too old to be able to change with the flow of technology.
I am also a former editor, from the old school, and still use two spaces after a period because – for the life of me – I cannot get my hands to stop typing that way.
I also feel it helps me personally with readability – not having everything so crammed together. But that’s just me. 
It ought to be TWO SPACES at the end of every sentence (except at the end of paragraph) and in some cases after a colon as well. The better books do this. Unfortunately, as someone else who is doing this says, for Webpages it requires the insertion of   after each sentence. Well done for bringing this up.