|The dash is a handy device, informal and esentially playful, telling you that you're about to take off on a different tack but still in some way connected with the present course — only you have to remember that the dash is there, and either put a second dash at the end of the notion to let the reader know that he's back on course, or else end the sentence, as here, with a period.
Use a dash [ — ] (or two hyphens [ -- ] on old-fashioned typewriters) or dashes as a super-comma or set of super-commas to set off parenthetical elements, especially when those elements contain internal forms of punctuation:
All four of them—Bob, Jeffrey, Jason, and Brett—did well in college.
In most word-processors, the dash is created by holding down the option key and hitting the key that has the underline mark above the hyphen. This can vary, though, from program to program. Usually, you get an en dash (see below) with the option + hyphen key, and you get the larger em dash (used more frequently) with option + shift + hyphen keys.
Do not use dashes to set apart material when commas would do the work for you. Usually, there are no spaces between the dash and the letters on either side of a dash, although the dash is frequently shown that way in documents prepared for the World Wide Web and e-mail for typographical and aesthetic reasons (because the WWW authoring and e-mail clients have little control over line-breaks).
In writing dialogue, the dash is used to show breaks in thought and shifts in tone:
"How many times have I asked you not to —" Jasion suddenly stopped talking and looked out the window.
"Not to do what?" I prompted.
"Not to — Oh heck, I forget!"
A dash is sometimes used to set off concluding lists and explanations in a more informal and abrupt manner than the colon. We seldom see the dash used this way in formal, academic prose.
Modern word processors provide for two kinds of dashes: the regular dash or em dash (which is the same width as the letter "M," — ) and the en dash (which is about half the width, the same as the letter "N," – ). We use the em dash for most purposes and keep its smaller brother, the en dash, for marking the space between dates in a chronological range: "Kennedy's presidency (1961–1963) marked an extraordinary era. . . ."; in time: 6:30–8:45 p.m.; and between numbers and letters in an indexing scheme: table 13–C, CT Statute 144–A.
The en dash is also used to join compound modifiers made up of elements that are themselves either open compounds (frequently two-word proper nouns) or already hyphenated compounds: the Puerto Rican–United States collaboration, the New York–New Jersey border, post-Darwinian–pre-Freudian theorems. The Gregg Reference Manual and the Chicago Manual of Style both recommend using the en dash whenever a compound modifier is combined with a participle as in "a Frank Lloyd Wright–designed building," "a White House–backed proposal," and "a foreign exchanged–related issue." A string of modifiers in a single compound, though, is joined with hyphens: hilarious, never-to-be-forgotten moments. If you are using an old-fashioned typewriter that cannot create an en dash, you can denote to your typesetter or editor that a hyphen is to be converted to an en dash by using a hyphen and hand-writing the letter "n" above it.
Some reference manuals are urging editors and publishers to get rid of the en dash altogether and to use the em dash exclusively, but en and em are still handy words to know when you're trying to get rid of those extra e's at the end of a Scrabble game. Finally, we use what is called a 3-em dash (or six typewriter hyphens) when we're showing that someone's name or a word has been omitted (perhaps for legal reasons or issues of taste):
Professors ______ and ______ were suspended without pay for their refusal to grade papers.
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