When should I use an em-dash, an en-dash, and a hyphen?

Question

I generally know how to use a hyphen, but when should I use an en-dash instead of an em-dash, or when should I use a hyphen instead of an em-dash?

Answer 1

A hyphen serves a few purposes. First, obviously, is hyphenation, as in "well-to-do" or "non-negotiable". Second, it is used to connect words which are broken due to justification of text, as in this example:

To be, or not to be, that is the question: Whether 'tis nobler in the mind to suf-

fer the slings and arrows of outrageous fortune, or to take arms against a sea

of troubles and by opposing end them.

Finally, hyphens are also used as negative signs for numbers, as in "-5".

En-dashes are used for ranges of numbers, as in "1–10". An en-dash may be employed with the alt-code ALT+0150 (hold the "Alt" key, type "0150" on the number pad, then release the "Alt" key).

Em-dashes serve as more emphatic commas for asides and appositives, though they serve more complex purposes in other languages, like Russian. An aside is a clause inserted into or at the end of another clause to provide further information on what was said immediately preceding or following the aside. For example:

John left the auditorium—it was eerily quiet after such a raucous assembly.

An appositive is a dependent clause inserted into or at the end of another clause to define or elaborate on the phrase that immediately precedes or follows it. For example:

Samantha climbed the rickety stairs to the dais—crumbling and weathered, despite its elaborate, decorative reliefs—and took her place alongside the others.

The em-dash may be employed using the alt-code ALT-0151. Some word processors will interpret two hyphens as an em-dash, like so: -- → —

It is important to remember that exotic punctuation such as the em-dash should be used sparingly. Abuse of em-dashes, semi-colons, colons, and the like is distracting to the reader.

Answer 2

A hyphen is for hyphenation: clumping words together or breaking them at the end of a typed line (which software will do for you).

"A two-year-old doesn't know how to use a hyphen."

An en-dash is used to express a range of numbers.

"The rules for dashes and hyphens can be found on pages 15–16."

An em-dash is for setting off a clause at the end or in the middle of a sentence.

"You may know how to use an em-dash—many people don't."

Or:

"Most people think they know how to use an em-dash—they are often wrong—but it's easy to get right."

(Note that it's always an independent clause that's set off, not just a phrase.)

A two-fer example would be:

"The words 'en-dash' and 'em-dash' are both hyphenated—they don't use a dash at all—and this is ironic."

In my experience, only the pickiest of editors will call you on these. I once had a professor complain about my using an em-dash and not setting off an independent clause. That was many years ago.