1. Use a dash to show a sudden change in the structure of a sentence or to indicate emphasis.
2. Use a dash singly or in pairs with word groups that summarize what has just been said or that provide details or examples.
3. Use a dash to link independent clauses when the second expands, develops, completes, or makes a surprising addition to the first (less formal than the colon).
4. Use a dash to emphasize parenthetical material. Use a comma for less emphasis than parentheses.

1. Use a dash to show a sudden change in the structure of a
sentence or to indicate emphasis.
- The old nations still live in the hearts of men, and love of the European nation is not yet born--if it ever will be.
(Raymond Aron, DAEDDALUS)
- We do not question the right of authors to spell, capitalize, and punctuate as they wish--provided they follow consistently a recognizable system. (John Benbow, MANUSCRIPT AND PROOF)
2. Use a dash singly or in pairs with word groups that summarize what has just been said or that provide details or examples.
- It takes a cataclysm--an invasion, a plague, or some other communal disaster--to open their eyes to the transitoriness of the "eternal order." (Eric Hoffer, THE TRUE BELIEVER)
- He was strongly in favor of peace--that is to say, he liked his wars to be fought at a distance and, if possible, in the name of God. (George Dangerfield, THE DEATH OF LIBERAL ENGLAND)
- Yesterday we shipped the merchandise--equipment invoiced over two weeks ago.
3. Use a dash to link independent clauses when the second expands, develops, completes, or makes a surprising addition to the first (less formal than the colon).
- And yet they had a thing in common, this oddest of odd couples--they both cared about the social graces. (George Frazier, ESQUIRE)
- In one respect, Welles was unique among the cabinet members-- he did not think himself a better man than the president.
(Margaret Leech, REVIELLE IN WASHINGTON)
- Janet Wilson is my favorite teacher--she is a math expert.
4. Use a dash to set off parenthetical material with greater emphasis than a comma but with less emphasis than parentheses.
- Dr. Lewis--a Ph.D. from Harvard--is the visiting professor this semester.
- Synonyms--words that have the same or nearly the same meaning--are found in the appendix of the usage text.
- The route they plan to take--Interstate 15 to Salt Lake and Interstate 80 to Laramie--enables them to visit their parents in Wyoming.