Chapter 3-7: The Dash (--) (—)
As noted in the section on commas, you can use a dash—also known as an em dash—at the beginning and end of parenthetical information. Usually, you will use dashes when you want to emphasise the information, but you might also use them if the parenthetical information is too long or abrupt to be set off with commas. Paired dashes enclose information that is closely related to the sentence and draws more attention to the information than paired commas would.
I think you would look fine wearing either the silk blouse—the one with the blue pattern—or the angora sweater. (abrupt interruption)
The idea of returning to the basics in the classroom—a notion which, incidentally, has been quietly supported for years by many respected teachers—is finally gaining some currency with school administrators. (lengthy interruption containing internal commas)
You can use a dash to conclude a list of elements, focusing them all toward one point.
Chocolate, cream, honey and peanut butter—all go into this fabulously rich dessert.
Dashes also mark sharp turns in thought.
We pored over exotic, mouth-watering menus from Nemo Catering, Menu du Jour, Taste Temptations, and three other reputable caterers—and rejected them all.
Word processors create an em-dash (or just "dash") from two hyphens (traits d'union) placed between two words, with no spaces.
The crowd clearly indicated their indifference to the provocative speech--an apathy that later came back to haunt them.
The dash can also act as a parenthetical aside that has more emphasis than two commas.
Peter chose to eat the sushi—knowing he was allergic to seafood—and then complained to the wait staff.