Thomas keeps asking about dashes, one of my guilty pleasures. I use way too many dashes! Anyway, he asked about en-dashes and em-dashes.
First, understand that this is a house style thing. That is, no matter what you do, the copy editor is going to "fix" it to fit the publishing house style or what the typesetting software interprets correctly. So that's a good thing to ask your editor about, just so you can put the correct one in to make sure that the copy editor "reads" it right, as a dash and not a hyphen.
But en vs. em isn't something that will earn you a rejection.
Just in case, though, here's the customary usage:
A hyphen isn't actually a punctuation mark as it's used within a word or term to join. A hyphen is a very short line (in typing it, you'd type - just once) which is generally used within a word or term to make two parts into one unit, like "half-soused." (There are, of course, other uses of the hyphen, but that's the most important.)
That means that the first part (before hyphen) and second part (after hyphen) go together and are to be regarded as one term. These combinations are usually called "coined terms" because the writer has "coined" the term by putting together two other words. The first modifies the second, and together they might modify another word.
The half-soused writer stumbled home to his blank pages.
I've been free-lancing my real estate articles.
The longer a term has been in the language, and the more common it is, the more likely you are just to make it a word and not a hyphenated "coined" term. So "halftime" and "lifestyle," which were once hyphenated terms, are now just words.
Hyphens and dashes are physically similar, and they both "join" things. But hyphens are used within words, and dashes within sentences and phrases.
En-dashes are longer than hyphens and shorter than em-dashes. (The "en" and "em" refer to the length of the line-- the "en" is as long as an "n" relatively, and the "em" is as long as an "m" character.) En-dashes are usually used for from-to ranges:
The open house is from 2–4 pm.
The Atchison–Topeka–Santa Fe railroad linked three major western towns.
Joan Parker, RIP. 1934–2010.
En-dashes are NOT two hyphens. Most word processors use "insert symbol" to put in the dash of sufficient length.
Em-dashes are longer than either the hyphen or the en-dash. In the days before word processors, we used to type the hyphen twice to make this. Some of us still do. :) It's usually possible in a word processor "auto-correct" to have an automatic substitution-- you type in the two hyphens, Mr. Word supplies a em-dash. Doesn't work in Blogger, however!
Em-dashes are used within a sentence to create a parenthetical (but without parentheses) to show an interruptive thought or aside.
Maybe -- alas, fond hope!-- she would call him back with news.
Douglas-- my first boss-- was exactly a year younger.
Notice that the first is truly interruptive, but the second is me being lazy-- I could use commas instead. But as you can see, I overuse dashes.
Also, an em-dash signifies when you're ending a sentence abruptly before the end. ("Fading out" is signified by an ellipsis-- ...)
She should call him back and-- No! She wouldn't humiliate herself that way!
On Donder and-- Donder? Are you there? Blitzen? Where are you guys?
Dashes are very seductive, as they can substitute for other punctuation marks (commas and periods, mostly). And so some of us-- not all of us!-- use them so we don't have to think about whether this is a restrictive modifier (no commas) or a non-restrictive (commas) or if this is more a statement or a question or an exclamation. I for one always have to go through everything I write and replace half the dashes with non-dash punctuation. (Here are some examples from a grammar book writer.) My first drafts look like Emily Dickinson verses, only without the profound thought and rhymes:
"Hope" is the thing with feathers—
That perches in the soul—
And sings the tune without the words—
And never stops—at all—
(Emily D probably didn't mean for her "end-line" marker to be set in type, but many editions of her work have all those dashes.)
So-- well. Use the symbol when you can (as when you're typing in the word processor), and understand in a submission, your purpose is to let the eventual copy editor know what she/he should put in right there. That is, you don't have to know the house style, but you do have to type this in a way that conveys your meaning to the copy editor who does know the house style.
Oh, and what about spaces before and after? Again, the copy editor will do that if the copy is going to typesetting. But in something like this-- a blog, I mean-- I usually put a space AFTER but not before the dash. That's because the "before" is being interrupted, so no pause (space), but "after" is resuming in a more deliberate way. I don't like the look of it all smushed together without spaces--like this--so I put the space in after the dash to separate the dashed-phrase from the real sentence.
HOWEVER, when you use a dash at the end of dialogue to signify an interruption, you probably don't want a space between the dash and the close quote, so:
"You have never respected my right to spe--"
"Yes, I have!" he protested.
And now back to our regularly schedule mullings. :)