How to Use a Dash

The dash (—) is a mark of punctuation used to set off a word or phrase after an independent clause or a parenthetical remark (words, phrases, or clauses that interrupt a sentence).
Don't confuse the dash (—) with the hyphen (-): the dash is longer.
William Strunk Jr. and E.B. White explained in "The Elements of Style":
"A dash is a mark of separation stronger than a ​comma, less formal than a colon, and more relaxed than ​parentheses."
There are actually two types of dashes, each with different uses:

• The em dash—also called the "long dash," according to Oxford Online Dictionaries—and
• the en dash, which doesn't have another name but falls between the hyphen and em dash in terms of length. The en dash is so named because it is approximately the equivalent width of the uppercase letter N and the em dash is roughly the width of an uppercase M.

Thomas MacKellar wrote in his 1885 book, "The American Printer: A Manual of Typography" :
"The em dash...is frequently used in particular works as a substitute for the comma or for the colon, and is found particularly serviceable in rhapsodical writing, where interrupted sentences frequently occur."
 He noted several specific uses for the dash, including:

• A sign of repetition in catalogues of goods, where it means ditto.
• In catalogues of books, where it was used instead of repeating an author's name.
• As a stand-in for the words to and till, as in chap. xvi. 13-17.

The last use would today be an en dash, which indicates a range.
 
The En Dash
The Associated Press (AP) describes how to use the shorter dash. Some styles call for en dashes to indicate ranges of dates, times, or page numbers, or with some compound modifiers. For example:

• He worked from 9–5. 
• She works from 8 a.m.–5 p.m.
• The festival will take place March 15–31.
• For your homework, read pages 49–64.

To create an en dash using a keyboard on a Windows-based system, hold down the Alt key and simultaneously type 0150.
To create this punctuation mark on a Macintosh-based system, hold down the Option key and press the Minus key [-]. 

According to the American Psychological Association you would use the en dash for:

• Items of equal weight (test–retest, male–female, the Chicago–London flight).
• Page ranges (in references, “...Journal of Applied Psychology, 86, 718–729”).
• Other types of ranges (16–30 kHz).

Angela Gibson, (MLA Style Center, a writing resource for the Modern Languages Association), says the organization uses an en dash when a single compound adjective is a proper noun, as in:

• Pre–Industrial Revolution city.

She notes that the MLA also calls for an en dash when a compound in the predicate position includes a proper noun:

• The crowd was Boris Becker–obsessed.

 
The Em Dash
The AP explains that these punctuation marks are used to:

• signal an abrupt change,
• set off a series within a phrase,
• start lists,
• Before attribution to an author or composer in some formats,
• After datelines.

 
AP style calls for a space on both sides of an em dash, but most other styles, including MLA and APA, omit the spaces.
On a Windows-based system, you can form an em dash on a keyboard by holding down the Alt key and typing ​0151.
To create the em dash on a Macintosh-based system, hold down the Shift and Option keys and press the Minus key [-], or alternatively, you can press the Hyphen key twice and press Space.

There are two basic ways to use an em dash in a sentence:

• After an independent clause: Author Saul Below, in "My Paris," provides an example of using an em dash after an independent clause:
  • "Life, said Samuel Butler, is like giving a concert on the violin while learning to play the instrument—that, friends, is real wisdom."

 

• To set off words and phrases: Writers have effectively used em dashes to shoehorn a parenthetical thought or remark into a sentence, as this quote illustrates:
  • "Copper Lincoln cents—pale zinc-coated steel for a year in the war—figure in my earliest impressions of money."
    —John Updike, "A Sense of Change," The New Yorker, April 26, 1999

 
The dash has sparked quite a debate among punctuation experts, writers, and grammarians.
"The dash is seductive," says Ernest Gowers in "The Complete Plain Words," a style, grammar, and punctuation reference guide. "It tempts the writer to use it as a punctuation-maid-of-all-work that saves him the trouble of choosing the right stop."

There is support for the dash:
"The dash is less formal than the semicolon, which makes it more attractive; it enhances conversational tone; and...it is capable of quite subtle effects. The main reason people use it, however, is that they know you can't use it wrongly."
—Lynne Truss, "Eats, Shoots & Leaves"

​There is also some opposition using the dash:
"The problem with the dash—as you may have noticed!—is that it discourages truly efficient writing. It also—and this might be its worst sin—disrupts the flow of a sentence. Don't you find it annoying—and you can tell me if you do, I won't be hurt—when a writer inserts a thought into the midst of another one that's not yet complete?"
—Norene Malone, "The Case—Please Hear Me Out—Against the Em Dash." Slate, May 24, 2011