Em dashes are what the non-copyediting world call dashes. Constructed from three hyphens, they are easy to use and easy to overuse.
1) Use an em dash when your sentence is going in a dramatically different direction from itself. “They say I have ADD, but they don’t understand—oh look! There’s a chicken.”
2) Use an em dash to elaborate on a point. “Desdemona hated to do her homework—especially algebra.”
3) Use an em dash as a strong comma, or to present an aside “Desdemona had to do her math homework—algebra, no less—before she could go out for ice cream.”
Long story short: The em dash is all about interruption and directional changes in sentences. Do not overuse them. They become a crutch, a way to make language more expressive or to control the flow or pace. The em dash is like perfume. A little perfume is gorgeous and alluring. A lot of perfume is off-putting and gross.
Those of you who are writing dialogue can use them more often than the rest of us.
Questions? Comments? Need a treatise on how Emily Dickinson used the dash? Other punctuation marks you’ve never understood? Do you hate me now?
Fangs and Clause 
I am a chronic em dash abuser, but I’m trying to get better — look, there’s a chicken!
Admitting it is always the first step.
“They become a crutch”
You say that like it’s a bad thing.
Wait till you hear my views on the use of italics for emphasis.
*gulp*
You caught me! I constantly have to go back to my work and remove these. I blame the scattered nature of a first draft = the perfect incubator for “dramatically different directions.”
Plus, the way I talk in real life is FULL of asides, complete with hand gestures to tell my conversation partner: “Hold on now, I will return in a second.”
So, is there a space around the em dash, or no? The font on your blog makes it hard to tell. I had always put a space on either side but my editor said that was wrong so I stopped doing it.
Next question: For dialog or a quotation, of course the quotation marks go on the outside of the end punctuation. But if you are using the quotes to single out a word—a ‘word’, you see—where does the quote mark go? Before or after the comma, or period as the case may be. I’ve seen it both ways and have never known which way is correct.
(I imagine you at the front of the classroom, wearing some sexy glasses and slapping your palm with a ruler.)
1. No space. The blog font does whatever the hell it wants to do.
2. Inside, unless you’re British. “That ’em dash,’ if you can call it that, looks suspiciously like a hyphen.”
3. I hope I’m wearing something nice.
I’m more comfortable with this chapter. But after reading your first two answers to Averil (rules I’ve gotten a headache trying to research), I think I’m getting more comfortable with breaking the rules and living with it. My goal now, is to have the page look okay to someone who kind of cares, but doesn’t know the specific rules any better than I do.
Here’s the story. There is not just one set of rules. I follow Chicago Manual of Style (maybe you should too, judging by your location ;), which is a standard among publishers and many academics. The Brits, for example, do things entirely differently, UNLESS they are following Chicago too. Chicago dictates serial commas, no hyphens with adverbs, and no spaces around em dashes.
In proofreading circles, at the stages in publication when changes to the text mean money spent, you start thinking about “egregious errors” and “copyeditor errors.” So I might notice that reddish-brown dog is hyphenated in three instances but not the fourth. This is a copyeditor error; that is, only copyeditors would notice! It’s not actually wrong. A misspelling is a regular-person error, that is, a regular person would notice it, and thus it is egregious and must be changed.
So one of the major things is consistency. If you put spaces around your em dashes, do it consistently.
Sin boldly. If we were all copyeditors I would be shit outta work.
Bless you for explaining that, and thank you! That’s what I’m trying for: consistency in my bold writing sins. I love the sound of that.
Who, me?
Yes, me. Oh yes, I need to go to Em Dash Abusers Anonymous.
Oh god also Italics Abusers Anonymous, for that matter.
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