A dash [ — ] is a form of punctuation used 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.
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 websites and e-mail for typographical and aesthetic reasons.
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 —” Jason suddenly stopped talking and looked out the window.
“Not to do what?” I prompted.
“Not to — Oh heck, I forget!”
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 Canadian–United States collaboration, the Ontario-Manitoba border. A string of modifiers in a single compound, though, is joined with hyphens: never-to-be-forgotten moments.
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.