Long dashes — what grammarians call em dashes — are dramatic. Those long straight lines draw your eye and hold your attention. But long dashes aren’t just show-offs. They insert information into a sentence and introduce lists. Short dashes — technically, en dashes — aren’t as showy as their wider cousins, but they’re still useful. Short dashes show a range or connect words when the word to or and is implied.
Long dashes in writing
A long dash’s primary job is to tell the reader that you’ve jumped tracks onto a new (though related) subject, just for a moment. Here are some examples:
After we buy toenail clippers — the dinosaur in that exhibit could use a trim, you know — we’ll stop at the doughnut shop.
Standing on one manicured claw, the dinosaur — delivered to the museum only an hour before the grand opening — is the star of the exhibit.
The information inside the dashes is off-topic. Take it out, and the sentence makes sense. The material inside the dashes relates to the information in the rest of the sentence, but it acts as an interruption to the main point that you’re making.
The words between a pair of dashes may or may not form a complete sentence. Fine. However, some people use only one dash to tack a complete sentence onto another complete sentence. Not fine! (Also, an issue you may encounter on standardized tests.) Here’s an example:
Wrong: The curator painted the dinosaur orange — everyone hates the color.
Right: The curator painted the dinosaur orange — everyone hates the color — because she wanted to liven the place up.
Also right: The curator painted the dinosaur orange; everyone hates the color.
Also right: The curator painted the dinosaur orange — a color hated by everyone.
The first example sentence is wrong because a dash can’t link two complete sentences. The second example is okay because a pair of dashes can surround a complete sentence embedded inside another complete sentence. The third example avoids the problem by linking the two sentences with a semicolon. The fourth example is correct because a dash may add extra information at the end of a sentence, as long as the extra information isn't a complete sentence. (A color hated by everyone isn’t a complete sentence.)
A dash’s second job is to move the reader from general to specific, often by supplying a definition. Check out the following examples:
I think I have everything I need for the first day of camp — bug spray, hair spray, sun block, and DVD player.
Everything I need is general; bug spray, hair spray, sun block, and DVD player are the specifics.
Louie said that he would perform the ugu-ug-ba — the ritual unwrapping of the season’s first piece of chewing gum.
The definition of ugu-ug-ba is the ritual unwrapping of the season’s first piece of chewing gum.
Long dashes may be fun to write, but they’re not always fun to read. For a little change of pace, dash a new idea into your sentence. Just don’t dash in too often or your reader will be tempted to dash away.
Short dashes in writing
If you master this punctuation mark, you deserve an official grammarian’s badge — very attractive at cocktail parties! Short dashes show a range:
From May–September, the convicts prune commas from literature written over the winter.
Short dashes also show up when you’re omitting the word to between two elements:
The New York–Philadelphia train is always on time.
Finally, a short dash links two or more equal elements when and is implied:
The catcher–pitcher relationship is crucial to the success of the Yankees.
Don’t confuse short dashes with hyphens.