The en dash is used for dates, time, and a range of numbers. It would be used in a sentence like this one: “Jim was looking at the files for the company’s 2018–2019 tax season.” Don’t use the en dash when you use words like “between” or “from.”
Em dashes are my favourite. As I often say, two em dashes offer storage space in a sentence, a place to put an extra thought or a clarification. For instance, I have taught grammar for many years—formally at Options and informally in my teaching work at UBC—and I still haven’t tired of speaking about this punctuation mark. See what I did there?
Em dashes can also bring the drama that parentheses lack. They’re emphatic. Look at the following sentence to see how the pair of em dashes emphasizes the content within:
“She had a lot to say about the issue—1358 pages’ worth, apparently—so we thought it best to let her weigh in during the planning meeting.”
Furthermore, dashes can be used in place of a colon, creating a meaningful pause before the end of the sentence is revealed:
Example: The bank robber regretted one thing—getting caught.
As you may have noticed, using two em dashes will remove the need for other punctuation marks. Watch as the sentences shift ever so slightly when I use em dashes:
#1 Grammar instruction, a necessary ingredient for effective writing, is taught increasingly less often in schools.
#2 Grammar instruction—a necessary ingredient for effective writing—is taught increasingly less often in schools.
So many of my favourite writers use the dash beautifully. Poet Emily Dickinson may be the most famous dash enthusiast, but it’s a much loved mark. Perhaps too loved? Over the years, there have been the occasional polemics against the em dash, arguing that they interrupt the flow of a sentence and allow sentences to grow into unreasonable lengths. These critics have a point: make sure your em dashes are used sparingly, so that their effect remains striking.
As always, think about what you’re trying to communicate and choose punctuation marks that fit. Parentheses and commas are more subtle, less eye-catching, than dashes and might be better suited to your message and sentence.