Functions of the Dash
The dash is one of the most effective punctuation marks of all. It can halt readers in their tracks—it makes them pay attention—as they read through your words of wisdom. Basically, the dash creates the biggest of the "punctuational pauses," the smallest being the comma; next, the parentheses; and finally, the dash. Though the dash does have other uses, showing up primarily in creative writing, it can help expository writers in five situations.
1. Explanatory or Defining Phrases
Set off explanatory or defining phrases with a dash.
Of the three punctuation marks producing a pause—the comma, the dash, the parentheses—the dash produces the most abrupt pause of all.
2. Parenthetical Material
Set off parenthetical material with a dash.
The dash can halt readers in their tracks—it makes them pay attention—as they read through your words of wisdom.
3. Introductory Defining Phrase
Set off an introductory defining phrase with a dash.
Em dashes, parentheses, and commas—these are the major punctuation marks used to create a pause.
You should use the dash to set off an appositive when commas might cause confusion:
Pauses in sentences—explanatory phrases, defining phrases, parenthetical material, and introductory defining phrase—prompt many writers to use the dash.
5. Sudden Break in Thought
Set off a sudden break in thought with a dash.
The committee's expansive logic—it went far beyond any previous decision—increased the available remedies rather dramatically.
Victory—that was her only goal.