I have always been bewildered by hyphens and dashes. They look similar but they aren’t. I did some research and the first thing that I learned was that dashes are almost never required by the laws of grammar and punctuation.

It’s also important to distinguish between dashes and hyphens. Hyphens are shorter lines (-); they are most often used to show connections between words that are working as a unit (for example, you might see adjectives like “well-intentioned”) or to spell certain words (like “e-mail”).

Think of dashes as the opposite of parentheses. Where parentheses indicate that the reader should put less emphasis on the enclosed material, dashes indicate that the reader should pay more attention to the material between the dashes. A single dash can emphasize material at the beginning or end of a sentence. For example:

After driving for thirty hours over several days, we finally reached out destination—Florida.

Two dashes can emphasize material in the middle of a sentence. Some style and grammar guides even permit you to write a complete sentence within the dashes. For example:

The protesters—they were all carrying signs—lined up on both sides of the streets for the arrival of the presidential candidate.

Two dashes can emphasize a modifier. Words or phrases that describe a noun can be set off with dashes if you wish to emphasize them. For example:

Sam—his ego deflated—walked out of the room.

You can sometimes use a dash to help readers see that certain words are meant as an introduction or conclusion to your sentence. For example:

To improve your health, you should carefully look at the food you eat—fast food, fatty fried foods, junk food, and sugary snacks.

Phrases that add information or clarify but are not necessary to the meaning of a sentence are ordinarily set off with commas. But when the phrase itself already contains one or more commas, dashes can help readers understand the sentence. This is a sentence just with commas.

Even the simplest tasks, washing, dressing, and going to work, were nearly impossible after I broke my leg.

When you add dashes, it is easier to read.

Even the simplest tasks—washing, dressing, and going to work—were nearly impossible after I broke my leg.

Finally, written dialogue is another instance where dashes are used. If a speaker suddenly or abruptly stops speaking, hesitates in speech, or is cut off by another speaker, a dash can indicate the pause or interruption. Here are some examples:

“I—I don’t know what you’re talking about.” 

Frank had an idea. “I was thinking—”
“I don’t care what you were thinking,” Carol interrupted.