An em dash is a type of punctuation, which is similar in appearance to a hyphen and takes its name from the point length of one em. As such, it is sometimes called an M dash or M–rule, though it is not uncommon for it to be referred to as a mutton. The em dash signifies a break or change of thought by the speaker or narrator. This dash can be used when other similar kinds of punctuation, such as parentheses, colons, and ellipses are not suitable. The proper formatting of an em dash largely depends on the guide of style and usage a particular editor or publisher follows.
The em dash acquired its name from its defined length. An em dash is the length of one em. Keeping with the specifications of font sizes, the length of one em is expressed in points. For example, an em in 12-point type is 12 points wide. Likewise, an em in 16-point type is 16 points wide.
Although a writer, editor, or publisher generally uses an em dash to signify a change or break of thought, there are several other more specific reasons. For example, the dash might be used to insert an aside within a sentence or at the end of a sentence. It can also be used to indicate an interruption in the narrator’s or speaker’s thoughts or speech. Sometimes, an em dash is used in the place of an ellipsis, such as when the narrator or speak cannot continue due to emotional reasons or being distracted. Typically, these dashes are used when the use of parentheses, a colon, or other type of punctuation is not appropriate or strong enough.
Similar to most kinds of punctuation, proper formatting of an em dash depends on the style authority. Both “The Chicago Manual of Style” and “The Oxford Guide to Style” state these dashes should be set closed, which means there are no spaces on either side of them. Other style guides, such as “The New York Times Manual of Style and Usage,” call for an open set, which means there are spaces on each side of the dash. Some writers prefer the open set to the closed one, as the dash of the closed set seems too long. Overall, the preferred style will depend on the piece of writing, as well as where and by which entity it is being published.