In Strunk and White's Elements of Style, the authors describe them as "stronger than a comma, less formal than a colon, and more relaxed than parentheses." They also say that dashes work best when ordinary punctuation seems inadequate. In other words, use them for special emphasis.
Unfortunately, I see em dashes littering prose nowadays the way spent fireworks litter the street in front of my house after the 4th of July. Open any published book and count the number you see on a single page. It is sometimes staggering. They've become so ubiquitous that they've nearly replaced commas entirely, not to mention other appropriate forms of punctuation like colons, parentheses, semi-colons (cue the dreaded sigh), and simple periods.
We won't get into the origins of the dash or how they differentiate between en dashes and regular ol' hyphens, as different style manuals and publishers will use either one for the same purpose. Let's just stick to the methodology. Writers often use em dashes because they want to write the way they speak. When someone tells a story aloud, they tend to interject additional information in mid-sentence, or they speak haltingly and with extra emphasis on certain parts to make sure we're getting the gist of what they're saying. It's completely natural, and it is the way our brains like to operate when dealing with massive loads of information that we have to streamline into something moderately coherent.
The problem is, it's very difficult to read the way people talk. Constantly breaking up sentences with parenthetical content is jarring for the reader. It's akin to maddening 5 o'clock rush hour city traffic. Stop and go, stop and go. Furthermore, the dash is often used incorrectly.
If you use them to insert related information in the middle of a sentence, the sentence must still make grammatical sense without it. In that way, the dash is exactly like a comma. For instance: John was not a fan of violence—the kind usually found in movies—not karate class. If you remove "the kind usually found in movies" from the sentence, we have: John was not a fan of violence not karate class. Not good. We have a run-on, but that is minor. We just don't know exactly what is modifying what. Those "nots" confuse everything.
Now, if you insist on keeping those infernal dashes in your sentence, the appropriate way to phrase it would be: John was not a fan of violence—the kind usually found in movies—though he enjoyed karate class. It still isn't perfect, but the thought is a bit clearer. We now understand John's preferences.
How would we better phrase this without dashes? Now, let's not always see the same hands. ;-) John was not a fan of movie violence, though he enjoyed karate class. Now we not only have a clear thought, but it reads smoothly and it knocked out 5 unnecessary words, which is a bonus.
As you read through your own work, look at those dashes as little stop marks and ask yourself whether your sentence demands them. First, is it even being used correctly in the grammatical sense? Does your sentence still make sense without the parenthetical info? If so, can you convey this thought and its intended emotion by using commas? If so, look for a more efficient way to combine your words so that the dashes can be eliminated altogether. Often you can link the content together via commas or semi-colons.
Better yet, if the sentence appears long and unwieldy, consider breaking it up into two separate thoughts. Sometimes you can gain more emphasis by just having a couple of short, staccato-like sentences standing side by side. If that still doesn't work, look at your vocabulary.
The strength of your idea usually hinges on the words you choose to express it rather than a punctuation mark. Rather than say: "She screamed—screamed loudly and forcefully—for help, and no one came," you can say "She shrieked for help, and no one came." You will not be able to remove all em dashes from your work, and I don't suggest that you do. Sometimes they are a dire necessity. I use them myself from time to time. It's just important to remember that the tools we often use to enhance our clarity can just as easily hinder it.
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