- To set off material you might put into a parenthetical expression but deserves more emphasis.
"Everything that went wrong -- from the peeping Tom at her window last night to the my head-on collision today -- we blamed on our move" (Hacker, 2007).
- To set off nouns or noun phrases that rename the earlier noun
"In my hometown, the basic needs of people -- food, clothing and shelter -- are less costly than in a big city like Los Angeles" (Hacker, 2007).
- To set up a list or a dramatic shift in tone
"Along the wall are the bulk liquids -- sesame seed oil, honey, safflower oil, maple syrup, and peanut butter" (Hacker, 2007).
"Kiere took a few steps back, came running full speed, kicked a mighty kick -- and missed the ball" (Hacker, 2007).
Yesterday we mentioned the em-dash. What we are referring to here is using 2 hyphens ( --). Most computer programs already have this in the formatting. If you have ever noticed when you put in a hyphen and then move on, how it increases in size. This is the em-dash.
It should be noted that in the case of the list, using a colon would be the better choice.