Dashes (em dashes and en dashes)

En and em refer to the traditional letter-equivalent widths of the dashes.

Use a long dash (em) to denote an abrupt change in thought or an emphatic pause, or to set off a series within a phrase. The firm hired 20 agents—an unprecedented step. These qualities—good humor, insight, compassion—would serve him well.

The principal use of the short dash, or en dash, is to connect numbers and, less often, words. With continuing numbers—such as dates, times, and page numbers—it signifies “up to and including” or “from … to”: 8 a.m.–5 p.m.; for the assignment, see chapters 14–16; the Rome–Paris train leaves at 3 p.m. (For related information, see Dates and Times.) The en dash is also used in open compound adjectives that precede a noun: an Academy Award–winning director; the post–World War II years.

A single word or prefix should be joined to a hyphenated compound by another hyphen rather than an en dash: non-tenure-track faculty; non-English-speaking peoples. If the result is awkward, try to reword: a cup that is two-thirds full instead of a two-thirds-full cup.

Note: These dashes are not a series of hyphens. To create en and em dashes:

En dash:
On a PC: Alt+0150
On a Mac: option+hyphen (press down both keys at the same time)
HTML and XML code: See the HTML chart in the Appendix 
Em dash:
On a PC: Alt+0151
On a Mac: shift+option+hyphen (press down these keys at the same time)
HTML and XML code: See the HTML chart in the Appendix