Many authors have trouble with dashes. It’s hard to know which one to use and when to use it. There are several different kinds, and they all have different usages.
Qualification: Since I deal with mostly United States fiction, my style guide is The Chicago Manual of Style (CMOS). Other style manuals have different rules for dashes, especially if you live in another area of the world, such as the UK or Australia. This international blog is actually based in the UK, so feel free to ask questions in the comments (letting me know where you’re located) and I’d be happy to give you individualized answers. The following information is taken from the sixteenth edition of CMOS, section 6.75–6.89.
The shortest dash is the hyphen. It takes up just one space and is used to hyphenate words, such as a surface-to-air missile or an Internet-based business.
The next shortest dash is an en dash. It’s called that because in the days of typesetting, it took up the space of the width of a capital letter N. Microsoft Word will give you an automatic en dash while typing if you type a hyphen with a space – on either side of it and keep going. Like that. It may also convert a double hyphen (–) into an en dash, but it doesn’t do it consistently. My example does not show a correct usage in the United States, though. Per CMOS, the en dash is used to connect dates and other numbers. As in, “I worked at TSRA in April –June of 2013.” In this usage, it replaces the word “to.”
The next dash is called an em dash, because it’s the width of a capital letter M. On a machine with Windows, you can insert an em dash by hitting Ctrl+Alt+Minus (on the number pad) or by typing three hyphens (—). An em dash can be used instead of a commas, parentheses, or colons. They are generally used in pairs, although if used at the end of a sentence, then perhaps only one shows, with the period (full stop) ending the sentence. Some examples—quoted from CMOS—are here:
My friends—that is, my former friends—ganged up on me. (Encloses a phrase not necessary to the sentence.)
Darkness, thunder, a sudden scream—nothing alarmed the child. (Sets off the series of nouns.)
“Will he—can he—obtain the necessary signatures?” asked Mill. (Indicates a break.)
“I assure you, we shall never—” Sylvia began, but Mark cut her short. (Indicates an interruption. A trailing off is indicated by an ellipsis.)
Only if—heaven forbid!—you lose your passport should you call home. (Encloses an unnecessary phrase with additional punctuation.)
You can see that em dashes and hyphens are the dashes most frequently used in fiction. In US style, none of the dashes have a space either before or after, or on one side but not on the other. These are some incorrect styles I see:
My friends – that is, my former friends – ganged up on me. (Space on both sides of an en or em dash.)
My friends– that is, my former friends –ganged up on me. (Space only on one side of the en or em dash, generally toward the unnecessary clause.)
Two other, longer dashes are used in non-fiction, so I’m not including them here. They’re called the 2-em and 3-em dashes.
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