Uses of the em dash
1. Em dashes set apart parenthetical phrases or clauses in a sentence. In this use, em dashes are similar to commas and parentheses, but there are subtle differences. For example, em dashes are used when a parenthetical remark contains an internal comma or would otherwise sound awkward if enclosed by commas. Perhaps a useful way to think of the em dash is as a pause or parenthesis with somewhat more emphasis than a comma and somewhat less than parentheses. Here are a few examples of em dashes used well for this purpose:
Steely Dan’s title track to FM—a justly forgotten, Robert Altman-inspired 1978 comedy that tries to pass off Foreigner, Foghat, and REO Speedwagon as paragons of rock rebellion—initially sounds like an extension of that movie’s middle-of-the-road sounds. [AV Club]
Since 2007, the consensus of the economic establishment—bankers, policymakers, CEOs, stock analysts, pundits—has been catastrophically wrong. [Slate]
Both Dagan and Diskin oppose military action against Iran unless all other options—primarily international diplomatic pressure and perhaps sabotage—have been exhausted. [Guardian]
In other cases, em dashes can go where parentheses would be too strong a break in a sentence, for example:
The president’s nephews—sons of his late brother—include Amar, the deputy director for national security … [New York Times]
2. An em dash can indicate a sudden break, an interruption, or a trailing off, for example:
HOWARD: … She’s totally unapologetic, she’s—
CHEW-BOSE: She’s everything. [Interview Magazine]
3. Em dashes can replace colons or serve as harder versions of commas (similar to semicolons). While parenthetical em dashes often operate in pairs (see the examples under the first point above), hard-comma em dashes often function alone at ends of sentences, for example:
The all-renewable energy sector is 30 years away — and always will be. [Salon]
It’s that time of year again—time for New Year’s Resolutions! [Pegasus Books]
Em dash typography
Publications make varying style choices when it comes to rendering the em dash. Some use the equivalent of three linked hyphens surrounded by a space on each side (like — this), and some omit the surrounding spaces (like—this). Others, like the online New York Times and most non-U.S. publishers, use an en dash (or the equivalent of two hyphens) surrounded by spaces (like — this). Some use an en dash with no spaces (like–this). Others, such as the BBC online, use a hyphen surrounded by spaces (like – this). The hyphen is traditionally regarded as a poor substitute for the dash, but given the relative ease of typing the hyphen, its use in lieu of the dash seems to be on the rise, and many see nothing wrong in it. Ultimately, it is a matter of editorial preference.
In Mac OS X, an em dash can be typed very easily by holding down the option and shift keys and typing a hyphen.
In Microsoft Word, an em dash can be typed with ctrl + alt + numeric hyphen. Note that this is only available using the hyphen on the numeric keypad not the main keyboard. Word (on a Mac or a PC) will automatically render an em dash when two hyphens are typed, unspaced, between words.