What kind of dash do I use?

 There are three kinds of dashes in modern computer word processing: the hyphen, the en-dash, and the em-dash. Here's a brief catalog of when and how to use each: 

The hyphen ( - ) is the small­est of these marks. It is also the only one with its own key: next to the 0. It has three uses.

1. It serves to break a word across two lines by syllables.(To maximize the readability of your text, it is recommended that you auto-hyphenate, particularly if you are using full justification. To auto-hyphenate in Microsoft Word 2007, click on the Page Setup Tab, and then the Hyphenation button.)
2. Some mul­ti­part words are spelled with a hyphen (topsy-turvy, cost-effective, bric-a-brac). But a pre­fix is not typ­i­cally fol­lowed with a hyphen (non­profit, not non-profit).
3. A hyphen is used in phrasal adjec­tives (commercial-speech restric­tion, estate-planning attor­ney, law-school grades) to ensure clar­ity.

Dashes come in two sizes — the en dash and the em dash. Just as the letter "m" is about twice as wide as the letter "n," the "em" dash ( — ) is about twice as wide as the en dash ( – ). Like the hyphen, they each have different purposes, and should not be used interchangeably.
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The en dash has two uses.

1. It indi­cates a range of val­ues, e.g., 1800–1912, 383 So. 2d at 452–455, Exh. A–E. Note: If you begin with "from," don't use an en dash, but use "to" instead, e.g., "from 1800 to 1912."
2. It denotes a con­trast between pairs of words, and is often used in place of "versus,"  e.g., conservative–liberal split, Fischer–Spassky match.
3. It is also used for joint authorship, e.g., Prosser–Keaton textbook.

Be care­ful when cit­ing a source like Local Rule 7-3. That gets a hyphen, not an en dash, because it’s the mul­ti­part name of a sin­gle rule, not a range of rules.

There are several ways to create an en dash:  

• With num lock on, type ALT + 0150 on the number keypad.
• In Microsoft Word, you can also type any word, then type "space, hyphen, space (" - "), then type another word (or any character) and then press space. The hyphen will elongate into an en dash.

The em dash is used for the following:

• To mark an interruption of a thought. E.g., "The last time I saw him—or so I thought—he was in England."
• It can replace the colon, e.g., "The company was awarded the largest contract ever for a prison—a 2048-bed facility."

There are several ways to create an em dash:

• Hold down the alt key and type 0151 on the number pad (make sure num lock is on)
• In Microsoft Word , you can also type two hyphens (minus signs) directly after a word, type another word (or any character) and then press space. The hyphen will elongate into an em dash.