Phineas P. Gage was an American railroad construction foreman now remembered for his survival of an accident in which a large iron — was driven completely through his head
A dash is one of several kinds of punctuation mark. Dashes appear similar to hyphens, but differ from them primarily in length, and serve different functions. The most common versions of the dash are the en dash (–) and the em dash (—).
The en dash, n dash, n-rule, or “nut” (–) is traditionally half the width of an em dash. In modern fonts, the length of the en dash is not standardized, and the en dash is often more than half the width of the em dash. The widths of en and em dashes have also been specified as being equal to those of the upper-case letters N and M respectively, and at other times to the widths of the lower-case letters.
(…)
The em dash (—), m dash, m-rule, or “mutton,” often demarcates a break of thought or some similar interpolation stronger than the interpolation demarcated by parentheses, such as the following from Nicholson Baker’s The Mezzanine:
At that age I once stabbed my best friend, Fred, with a pair of pinking shears in the base of the neck, enraged because he had been given the comprehensive sixty-four-crayon Crayola box—including the gold and silver crayons—and would not let me look closely at the box to see how Crayola had stabilized the built-in crayon sharpener under the tiers of crayons.
It is also used to indicate that a sentence is unfinished because the speaker has been interrupted. For example, the em dash is used in the following way in Joseph Heller’s Catch-22:
He was Cain, Ulysses, the Flying Dutchman; he was Lot in Sodom, Deirdre of the Sorrows, Sweeney in the nightingales among trees. He was the miracle ingredient Z-147. He was— ”Crazy!” Clevinger interrupted, shrieking. “That’s what you are! Crazy!” ”—immense. I’m a real, slam-bang, honest-to-goodness, three-fisted humdinger. I’m a bona fide supraman.”
Similarly, it can be used instead of an ellipsis to indicate aposiopesis, the rhetorical device by which a sentence is stopped short not because of interruption but because the speaker is too emotional to continue, such as Darth Vader’s line “I sense something; a presence I’ve not felt since—” in Star Wars Episode IV: A New Hope.
