My old school training was to put two spaces after a period. But it seems that the world has moved on without telling me. Now one is the rule. True?
Is this true internationally? My South African boss insists on two, but he is older than me, so maybe he just never unlearned that.
Hmmm???
-Tcat
It’s not really grammar, it’s typographical style.
I used to work as a typographer, and I was told by the boss that for clarity, it used to be necessary to insert two spaces to separate sentences, but since modern DTP software elongates a single space after a period/full stop, one will now do.
Of course, the Web is also changing things, with one space the norm, and I think that’s really just fashion. I personally only ever use one.
I always learned (and still use) two spaces after a period. According to a computer/keyboarding teacher at the school where I teach, though, one is now acceptable in business situations. I can’t get myself to do it, though. Too many years using two. Plus, I think it looks better with two.
Typucally there has always been two spaces after a period. Some of the rules are eveolving though with e-mail and other digital output – our proofreadres think it has to do with typesetting and just a general flow of text to make large paragraphs legible (easier for the eye to follow along.)
I’ve started getting realy used to seeing onl one space after the period because most e-mail programs don’t give you the choice. I’m also getting used to paragraphs that are not indented for the same reason.
I caused a raging debate amoung our French proofreaders because in French you often see a space before some punctuation. So one would see either “Ça va ?” or “Ça va?” Meanwhile, there was no space before the period. “Oui, ça va.” None of the proofreaders could find any rule that would explain why there was a space before question marks and exclamation points, though they agreed “yeah, as far as I can remember, it’s kinda always been that way.”
Though we may be very wrong – this is a guess, remember – it seems that some of the conventions of space around punctuation may have come from “ye olden days” when typesetters had to line up all those little metal blocks. At least that’s the only explanation anyone could think of.
Hope someone comes to correct me if I’m wrong.
I should point out that HTML condenses any number of consecutive spaces down to one, so though nineiron is using two here, only one is being displayed.
The reasonably respectable Chicago Manual of Style seems to be fairly unambiguously in favor of one space:
It’s my opinion that if you’re consulting a style manual for how to take your notes, you’re nuts. 
An earlier thread on this subject:
http://boards.straightdope.com/sdmb/showthread.php?threadid=83668
In typed stuff I always use two spaces. When it comes to printing or DTP, the program I use puts in one space by default, but if the line is loose it first adds extra space after periods (and ! and ?) before adding space elsewhere. The results look pretty good.
As a former book editor, I can tell you that uniformly putting one space after a period is nothing new. Look at the vast majority of English-language books published fifty years ago, or a hundred years ago, and you’ll still see one space after a period, not two. But because almost all book text is “justified” (stretched horizontally so that the left and rights edges of the text run evenly to the margins), spaces between words and spaces between sentences can sometimes appear to be one and a half or two spaces wide.
Putting two spaces after a period is a relic of typewriters and their monospace typefaces (typefaces where the center of each letter was the same distance from the center of the next letter, no matter how wide or narrow the letters were). Two spaces was thought to be necessary to break up the visual monotony of such spacing, and to be a visual enhancement of the period’s function.
Putting two spaces after a period introduces an unattractive “gutter” of space between sentences that snakes down a page of typeset text.
Walloon is correct. Two spaces is from typewriters, not typography. Typography has always had one, though it was adjusted if the text was justified.
Now, since computers use proportionally spaced fonts, we’re all doing a limited form of typography. So no one needs two spaces. That applies, by the way, to all situations. There is never a need to hit the space bar twice in a row anywhere in a modern document.