Em-dash

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The em-dash (—), also known as the em rule, indicates a sudden break in thought—a parenthetical statement like this one—or an open range (such as "John Doe, 1987—"). 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 period is too strong and a comma too weak.

The em-dash is defined as one em in width. An em is equivalent to the point size. Thus, in 9-point type an em is 9 points wide (as well as 9 points high) while the em of 24 point type is 24 points wide (and high), and so on. By definition, this is twice as wide as the en-dash in any particular font.

Monospaced fonts such as Courier or DP Sans Mono that mimic the look of a typewriter have the same width for all characters. Thus, they have only a single hyphen glyph, so it is common to use two monospace hyphens strung together--like this--to serve as an em-dash.

That is how it is done at DP. Since every project has a plain text version, the end result will be in a monospaced font. Thus the term em-dash is used to mean two hyphens --, as explained more fully in the Dashes, Hyphens, and Minus Signs section of the Guidelines.

Sometimes at DP the term "em-dash" is used to refer to a dash which is twice as long as an em-dash (like this: ——), but this would be more properly termed a "two em dash." At DP it is often simply called a "long dash."

This information was shamelessly copied, in part, from the Em-dashes section of wikipedia.