In the course of your reading adventures, you might have happened across a mysterious symbol—a kind of elongated hyphen. Did you see that? One just slipped by!
What is this symbol? What does it mean? And most importantly, when can you use it to spice up your own writing?
Dear reader, this—is an em dash. It’s called an em dash because it is approximately the width of an “m.” This distinguishes it from its shorter variant, the en dash, and from the all-too-familiar hyphen.
Usage of the em dash is similar to that of the comma, semicolon, or colon: it introduces a pause in your prose. Upon learning this, you may be tempted to substitute most or all of your pauses with this typographical marvel—but to do this is to squander its power.
In general, most pauses can—and should—be handled by a comma. Commas are small pauses; semicolons and colons longer; sentences longer than that; and paragraphs are the longest pause of all, at least, outside of sequels. An em dash is deliberately disruptive. It represents a break in thought, a kind of shoving to one side of a phrase or clause.
Overuse of em dashes can normalize their pauses and suggests that the author cannot work out which punctuation is appropriate for a given situation. If you or a fellow writer suffer from this affliction, try replacing your em dashes with commas, then go back and work out which of those commas really need to be replaced by colons of the full or semi variety. Finally, for moments that truly deserve a longer pause or a clearer break, put your precious em dashes back in—but sparingly.
In most writing software, em dashes can be typed by typing two dashes together (--). If you find yourself in an em dash emergency and your app cannot produce one, copy it from a browser test page.
One final note: em dashes are typically associated with informal writing. If you are writing an academic paper, documentation, or something else where your writing needs some gravitas, avoid em dashes altogether. They can always be replaced by commas, colons, semicolons, separate sentences, or in a pinch, parenthetical phrases (but even these tend to undermine formality).