Found in: Punctuation
There are two kinds of dashes—of different length and for different purposes—in addition to the hyphen.
The em dash (—) is the true dash, used for parenthetical remarks or abrupt changes of thought, epigraphs, and datelines. Because there was no dash character on a typewriter keyboard, dashes were traditionally indicated by two hyphens (--). Computers offer the ability to produce a proper dash.
Newspapers traditionally used a space on each side of a dash, and AP still requires such spaces. However, letterspacing is the responsibility of the designer (of the typeface and of the publication), and it is not necessary to routinely add such spaces. Columns does not use spaces around dashes.
The em dash is named for the amount of letterspace, that of a capital M, that it occupies in a line of type set in the particular typeface.
The en dash (–) is shorter than an em dash (it takes up the amount of letterspace occupied by a capital N) and longer than a hyphen. It is used for continuing or inclusive numbers or words (range constructions: pages 7–10; Jan. 5–9; E-P; Monday–Friday), but not when the word “from” is actually used (1968–72 or from 1968 to 1972 , never from 1968–72).
An en dash also is used in place of a hyphen in a compound adjective when one of the elements is an open compound (post–Civil War period; Peabody Award–winning program, Athens–Clarke County government ), when referring to one campus of a multi–campus university (University of Wisconsin–Madison ), when combining two equal elements (Paris–Rome train ) or when combining two hyphenated compounds ( quasi—public–quasi–judicial body).