More on this later, but I have to get to word out since apparently there's still the misconception: unless you are still using your grandmother's typewriter, you only put ONE space—not two—after a punctuation mark when ending a sentence.
I have to admit, I'm surprised by the response to this little guy! I even got an email via the Facebook feed. Good times. Allow me to explain.
The Short Version: Spiff's comment points to the heart of the issue. Each character on a typewriter took up the same amount of space on the page. A period was just as wide as a capital M, just with more room around it. So to make it clear when the sentence was over, you had to put two spaces. This has been replicated in "monospaced" fonts, like Courier.
Most modern typefaces are "proportional", meaning that each character takes up an amount of space proportional to it's size. A period takes up a tiny bit, a cap M a lot. So one space after a period is "big" enough to show the end of one sentence and the start of the next.
The Long Version: Funny that motormouth should mention it, because I'm about to quote at length from the very first chapter of The Mac Is Not A Typewriter, which was first published in 1990. I have a first edition*, I'll have you know (2nd edition here). Oh, and don't let the "Mac" part of the title throw you. It applies to all computers.
One Space Between Sentences
Use only one space after periods, colons, exclamations points, question marks, quotation marks—any punctuation that separates two sentences.
What? you say! Yes—for years you've been told to hit two spaces after periods, and on a typewriter you should. But this is no typewriter.
On a typewriter, all the characters are monospaced; that is, they each take up the same amount of space—the letter i takes up as much space as the letter m. Because they are monosapced, you need to type two spaces after periods to separate one sentence from the next. But...
On a Macintosh (unless you're using the fonts Monaco or Courier, which are monospaced just like a typewriter and what would you want too sue those for anyway**) the characters are proportional; that is, they each take up a proportional amount of space—the letter i takes up about one-fifth the space of the letter m. So you no longer need extra spaces to separate the sentences.
Of course, this one-space rule applies just as well to the spacing after colons, semi-colons, questions marks, quotation marks, exclamation points, or any other punctuation you can think of. Yes, this is a difficult habit to break, but it must be done.
Take a look at any magazine or book on your shelf—you will never find two spaces between sentences.
Maybe next week we'll discuss the difference between hyphens, dashes, en dashes, and em dashes. And the Oxford comma***!
**There are good reasons to use these fonts. She's making a joke, people!
***Screw you, Vampire Weekend!
Wow! You spend one day moving to a new cube and the most random post in the world ends up starting a turf war!
NP Steve, I hope our "pompous"-ness doesn't drive you away. :-)
The thing about all these typographic elements, is that they exist so that they can be invisible. That is, when properly used, they aid in comprehension and readability without the reader ever thinking about what they just saw.
These weren’t designed for kicks and giggles. They help you communicate clearly and concisely. Whether you're writing professionally (as several of the commenters do) or for a business presentation or a school paper or a blog post (ha! No doubles on the net!), you never want your reader to stop reading and start noticing what you've written. Does that make sense? Lemme give an example.
When typing on a computer, using two spaces after a period makes a noticeably large gap. A gap bigger than anything else you'll find on that page. Because you're not supposed to use two spaces. Modern typefaces (fonts, if you will) are DESIGNED and INTENDED for you to only have to press the space bar once. So by using two spaces, you actually work against your computer, word processor, and typeface designer.
The result is your reader is left with big visual potholes all over the page. Because all the thousands of people that made that computer of yours possible never intended for you to press the spacebar twice. And every time a reader comes to the end of a sentence, it’s like a tire dropping in a pothole. BANG!
Now I admit, my word picture is a bit more intense then the actual reading experience. But the idea is the same. There is an unexpectedly large drop (or break) in the visual flow. And when you’re writing, you want to minimize anything that would take your reader “out” of what they’re reading. You want to stay in their head, not have them stay on the page.
There was a reason why we did two spaces with typewriters. And there's a reason we do one now. There's not some "self-appointed arbiter" making things up. It's legit. It's not "nonsense" and it does very much have to do with flow and appearance. And it is totally the way the “pros” do it. Open any book or magazine and check. Even the Bible uses a single space after periods, so consider it inerrant. ;-)
The same concept applies to hyphens, en dashes and em dashes. There are specific reasons and times when you use them (future post!). They have a purpose and a design. But the reader just sees them and automatically knows what they mean. They don't even have to think about it.
This is how type should work—it should be invisible. As Elvis Costello once said, "It had to be subtle if it was to have any meaning at all."