Although they are each a simple horizontal line, hyphens and the various dashes have their own appearances and specific uses. The shortest and most common is the hyphen, which is used for clarifying open compounds (such as "four-armed villains," as opposed to "four armed villains"), for separating number groups in phone and Social Security numbers, and for connecting written-out fractions like three-quarters.
Although many people call the hyphen a "dash," dash refers to another group of horizontal lines in text. The two most common dashes are the em dash and the en dash.
Em dash
Em dashes — so called because they are (at least historically) the width of the character m — are used for emphasis or interruption. They can be used on their own or in pairs to offset a word or phrase:
Many people seek help from naturopathic medical professionals — those who emphasize using diet, exercise, meditation, and other tools for improving health.
A vast amount of serotonin — about 95 percent of the body's total — is produced in the digestive system.
The double hyphen (--) is sometimes used in place of the em dash. You may notice that word-processing programs automatically convert double hyphens into em dashes for you. However, since the programs aren't infallible, some stray double hyphens may remain, so if you're copyediting on hard copy, you'll want to mark each em dash no matter how it appears in the text. The production editor will then make the global change to convert everything to proper em dashes. If you're proofreading, you need to make sure the change has been made and note any stray double hyphens that need to be corrected. (If you're copyediting electronically, just do a global search-and-replace to convert all double hyphens to em dashes — or vice versa, if that's what the production editor wants.)
You can create an em dash in Microsoft Office software (among others) by pressing Ctrl+Alt+– on the Number Pad. To do a find and/or replace, plug the code ^+ into either the Find What or Replace With text boxes to search for or replace with an em dash.
Em dashes also precede quotation attributions:
Poetry is a deal of joy and pain and wonder, with a dash of the dictionary.
— Kahlil Gibran
You may encounter a supersized first cousin of the em dash if you work on texts that include bibliography sections. The 3-em dash (which is the width of three m's in the corresponding font) most frequently appears in a bibliography when an author's name is cited more than once. Repeat listings by that same author feature a 3-em dash rather than the author's name again.
En dash
En dashes — dashes that are the width of the character n — have multiple uses. They're used to connect two items (usually numbers) that designate a range:
We submitted chapters 10–12 well after midnight.
I left at halftime with the score stuck at 3–1.
The January–February issue is due on newsstands tomorrow.
They take the place of hyphens when one part of an open compound is made up of two words:
The author is a Nobel Prize–winning physicist.
In the above example, Prize and winning are joined, but Nobel is just floating out there. The en dash works a little harder than a hyphen to show that the word Nobel is included in the open compound.
En dashes may also be used to indicate that tension or opposition exists in a relationship, or to show a direction of movement:
The article supports the tax–spend hypothesis: Tax revenues determine government spending.
The New York–Washington, D.C. run takes about three hours by train.
Admittedly, the idea of tension can be difficult to pinpoint. Definitely check what the house style or the preferred style guide has to say about en dashes.
In the Microsoft Office Suite of applications (and many others as well), you can create an en dash by pressing Ctrl+– on the Number Pad. To do a search and/or replace, the code ^= represents the en dash.
