What are the correct ways to use the long dash (em-dash)?

Question

So I've seen these (—) used a lot in writing (mostly in fiction), but I'm still unsure of the correct way to use them.

I've seen them used to define something.

I have several things in my backpack — pencils, erasers, and papers.

When you're defining something, do you have to be exact? For instance, I probably have more than just pencils, erasers, and papers in my backpack, so do I have to write everything down when using an em dash to define something? Also, when I'm defining something, can it be in between the sentence, instead of at the very end (this is very uncommon though, right?) e.g. I have several things — pencils, erasers, and papers — in my backpack.

I've seen them used to include ideas in between.

Something I'm confused about is the type of idea you put using the em dash. Is it something that's important for the reader to know, or simply included to enhance your writing. Can someone please elaborate on this one (probably the most confusing one for me...)

I've seen them used to end a sentence abruptly.

This usually gives the feeling of suspense to dialogue.

"What's going on? Where am —"

I've seen them used to answer questions or problems or disputes.

"Johnny killed Harris."

"No, it wasn't Johnny — Harris killed himself."

Answer 1

The em dash (and arguably the en dash when set off by spaces) is perhaps the freeest punctuation mark today and the closest contemporary punctuation to original punctuation marks which only indicated when a writer thought a pause would be helpful. (Ugh, can't find the article I was just reading yesterday. Will share.)

The Chicago Manual of Style says the em dash

allows, in a manner similar to parentheses, an additional thought to be added within a sentence by sort of breaking away from that sentence.

This is a very free definition. Essentially whenever the writer wants to include any additional information, they can use an em dash. (The CMOS article also has an interesting bit about tying the length of a dash to the amount of work it is doing.)

An em dash, then, is not specifically used for defining so you do not need to include everything in the backpack. Of course, for clarity's sake, you could include "among other things" or "and more".

As for you question about whether a clause separated by an em dash can be in the middle of the clause, certainly! Look back and see the CMOS refers to it as similar to parentheses. Examples of such usage abound.

More from CMOS:

Em dashes also substitute for something missing.

This usage explains the dialog part of your question.

In fact, em dashes can find an even greater role in dialog: they can replace quotation marks, though this is less common in English than in other languages. See wikipedia.

For a fuller understanding of the em dash, I quote the CMOS Online in full:

The em dash has several uses. It allows, in a manner similar to parentheses, an additional thought to be added within a sentence by sort of breaking away from that sentence—as I’ve done here. Its use or misuse for this purpose is a matter of taste, and subject to the effect on the writer’s or reader’s “ear.” Em dashes also substitute for something missing. For example, in a bibliographic list, rather than repeating the same author over and over again, three consecutive em dashes (also known as a 3-em dash) stand in for the author’s name. In interrupted speech, one or two em dashes may be used: “I wasn’t trying to imply——” “Then just what were you trying to do?” Also, the em dash may serve as a sort of bullet point.

Answer 2

As far as I know and remember from college, the number 1 rule for using the long dash is for when the author wants to give you extra information. Thus, you may delete whatever between the long dashes, and the sentence will hold perfectly grammatical.

I believe this will explain your first two usages: to define something and to include something.

And this other use I have almost forgotten, but as can be seen in your last two examples, it means a pause in the writer's sentence or, if in a quote, the speaker's sentence being uttered. So this shall help explain the last two usages.