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:
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:
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:
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:
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:
The Em Dash
The AP explains that these punctuation marks are used to:
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:
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
Meaning and Sentence Structure
"People are probably not as aware of sentence structure as they are of sounds and words, because sentence structure is abstract in a way that sounds and words are not . . . At the same time, sentence structure is a central aspect of every sentence . . . We can appreciate the importance of sentence structure by looking at examples within a single language.
For instance, in English, the same set of words can convey different meanings if they are arranged in different ways. Consider the following:
• The senators objected to the plans proposed by the generals.
• The senators proposed the plans objected to by the generals.
The meaning of [first] the sentence is quite different from that of [the second], even though the only difference is the position of the words objected to and proposed. Although both sentences contain exactly the same words, the words are structurally related to each other differently; it is those differences in structure that account for the difference in meaning."
Fernández, Eva M. and Cairns, Helen Smith. "Fundamentals of Psycholinguistics." Wiley-Blackwell, 2011