Dashes are a friendly way to take a break from what you started to say and interject something different. Commas are polite, reminding you to pause, but not calling attention to themselves. Dashes are bold and casual.
Dashes can take the place of commas when you write a long appositive or parenthetic expression.
- Henrietta Whitcomb — my great-grandmother’s secret, lifelong roommate — mowed the lawn every weekend rain or shine.
- The old station wagon — as I have said to you over and over again — won’t last another winter.
They can also signal an interruption in thought or speech.
- The old man took me for another interminable, breakless walk through the fecund streets of the neighborh — Squirrel!
The poet Emily Dickinson is famous for her use of dashes in lieu of other punctuation:
I’ll tell you how the Sun rose –
A Ribbon at a time –
The steeples swam in Amethyst
The news, like Squirrels, ran –
The Hills untied their Bonnets –
The Bobolinks – begun –
Then I said softly to myself –
“That must have been the Sun”!
In The Elements of Style, Strunk + White caution us to use dashes discerningly. If a plainer punctuation suffices, use it instead.
- Bob, my roommate, mows the lawn each Saturday.
- The old station wagon, as you know, won’t last the winter.
M-dash, N-dash, and Hyphen
Three dash-type punctuation forms exists. The m-dash, the n-dash, and the hyphen. This article explains how to make m- and n-dashes on your computer (easiest for Mac users) as well as what style guides say about spacing before and after.
Use the m-dash as described above.
Use the n-dash to mean “to” or “through” as in the movie is from 1:00–2:00 pm, or we go to school Monday–Friday.
Use hyphens in special numbers such as phone numbers and Social Security Numbers. Use them in hyphenated words. And use them when you are combining adjectives to describe a noun:
- The three-year-old boy climbed into the pig pen before his mom could stop him.