Rhetoric and Composition/Hyphens and Dashes

Dashes ("—", "–") are used to mark an interruption within a sentence, while hyphens ("-") are used to join two parts of a compound word, or to indicate that a word has been split at the end of a line. A dash is approximately as long as two hyphens.

Dash
Dashes are minus-shaped characters used to mark an interruption within a sentence. They are used in much the same way as parentheses.

There are two kinds of dashes: em dashes looking like "—", and shorter en dashes looking like "–". It is usual to either use em dashes without spacing, or en dashes with spacing, but not em dashes with spacing, as shown in the following two examples:
 * Example: Three unlikely companions—a canary, an eagle, and a parrot—flew by my window in an odd flock.
 * Example: Three unlikely companions – a canary, an eagle, and a parrot – flew by my window in an odd flock.

In most word processors, there is a keystroke combination that will produce an em-dash:
 * Windows: em-dash: Alt-0151.
 * Windows: en-dash: Alt-0150.
 * Mac: em-dash: shift-option-dash.

Hyphen
A hyphen joins two parts of a compound word.
 * Example: governor-elect, twenty-five, half-baked.

Hyphens can also be used to make compound words more understandable. Consider these words:
 * Man-eating dog
 * Man eating dog

The first example describes a particular type of dog (man-eating). The second example, alas, suggests that a man is eating a dog.

Or consider the case of the flaming-red pickup truck, as opposed to its more alarming cousin, the flaming red pickup truck.

In general, if the first of two adjectives is describing the second, and not the noun following, you should use a hyphen: deep-blue water, good-tasting hamburger, happy-faced child.

Width Difference
The em dash is roughly as wide as two hyphens. In the days of typewriters, it was actually written as two hyphens. An en dash is shorter than em dash while wider than a hyphen.