Either my boredom — or my geekery — has reached new heights. Am sitting here in the dark with an ice pack on my foot reading articles online about punctuation.
Yes, I am that much of a nerd. (I read the dictionary for fun...)
But! In case you are ever on Jeopardy, did you know:
The em dash (—), or m dash, m-rule, etc., often demarcates a parenthetical thought—like this one—or some similar interpolation.
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 superman."
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 have not felt since—" in Star Wars Episode IV: A New Hope.
The term em dash derives from its defined width of one em, which is the length, expressed in points, by which font sizes are typically specified. Thus in 9-point type, an em is 9 points wide, while the em of 24-point type is 24 points wide, and so on. (By comparison, the en dash, with its 1-en width, is ½ em wide in most fonts.)
The em dash is used in much the way a colon or set of parentheses is used: it can show an abrupt change in thought or be used where a full stop (or "period") is too strong and a comma too weak. Em dashes are sometimes used in lists or definitions, but that is a style guide issue; a colon is often recommended for use instead.
According to most American sources (e.g., The Chicago Manual of Style) and to some British sources (e.g., The Oxford Guide to Style), an em dash should always be set closed (not surrounded by spaces). But the practice in some parts of the English-speaking world, also the style recommended by The New York Times Manual of Style and Usage (due to the narrow width of newspaper columns), sets it open (separates it from its surrounding words by using spaces or hair spaces (U+200A)) when it is being used parenthetically. Some writers, finding the em dash unappealingly long, prefer to use an open-set en dash. This "space, en dash, space" sequence is also the predominant style in German and French typography. See En dash versus em dash below.
In Canada, The Canadian Style [A Guide to Writing and Editing], The Oxford Canadian of Grammar, Spelling & Punctuation, Guide to Canadian English Usage [Second Edition], Editing Canadian English Manual, and the Canadian Oxford Dictionary are all defined NO SPACE before after these Em Dash marks when they are inserted between words, a word and numeral, or two numerals.
Monospaced fonts (such as Courier) that mimic the look of a typewriter have the same width for all characters. Some of these fonts have em and en dashes which more or less fill the monospaced width they have available. For example, "- – — −" will show as a hyphen, en dash, em dash, and minus in a monospace font. Typewriters often only have a single hyphen glyph, so it is common to use two monospace hyphens strung together--like this--to serve as an em dash.
When an actual em dash is unavailable—as in the ASCII character set—a double ("--") or triple hyphen-minus ("---") is used. In Unicode, the em dash is U+2014 (decimal 8212). In HTML, one may use the numeric forms — or —; there is also the HTML entity —. In TeX, the em dash may normally be input as a triple hyphen-minus (---). On any Mac, most keyboard layouts map an em dash to Shift-Option-hyphen. On Microsoft Windows, an em dash may be entered as Alt+0151, where the digits are typed on the numeric keypad while holding the Alt key down. It can also be entered into Microsoft Office applications by using the Ctrl-Alt-hyphen combination.
Fascinating. I love it! And now you know!