Em dashes (—) are used less as scientific notation and more as a stylistic tool in creative writing to place emphasis and direct the reader’s attention to specific parts of text. Because of this, within academic writing, em dashes are more commonly used in literature, the humanities, and social science.

In situations where a comma might be used to initiate a list of items, we can replace the comma with an em dash to distinguish the individual elements as standalone list items that invite attention. As a result, it helps in either breaking up a lengthy sentence into smaller pieces or emphasizing the more important information in a sentence without compromising flow. The same rule can be applied to situations where we might use parentheses to set off a list of elements. However, parentheses are usually reserved for encapsulating nonessential or optional information in the sentence that would still make sense should the reader choose to ignore the content within the parentheses.

Often, it’s difficult to distinguish between correctly using an em dash and when to simply use a comma. In general, it helps to think that commas are primarily used in conjunction with the word ‘which’. We can use em dashes to emphasize expressions of text or show a shift in tone:

 

Correct examples for emphasizing a sub-segmentIncorrect example
All four of them—Steve, Hugh, Rowan, and Rick—were Melbourne University alumni.

All four of them (Steve, Hugh, Rowan, and Rick) were Melbourne University alumni.

Use parentheses when the names listed here is optional or not essential information.
All four of them-Steve, Hugh, Rowan, and Rick-were Melbourne University alumni.

Incorrect because it uses hyphens, which are too short.
The system, which was installed on Thursday, has already broken.The system—which was installed on Thursday—has already broken.

Incorrect because it uses an em dash in conjunction with ‘which’.

The system-which was installed on Thursday-has already broken.

Incorrect because it uses hyphens, which are too short.

The system (which was installed on Thursday) has already broken.

Incorrect because parentheses are primarily used to denote material that is optional or nonessential to the sentence context.
Moore, B.D., Wallis, I.R., Palá-Paúl, J., Brophy, J.J., Willis, R.H. & Foley, W.J. (2004) Anti-herbivore chemistry of Eucalyptus—cues and deterrents for marsupial folivores. Journal of Chemical Ecology, 30, 1743–1769.

This sentence contains examples of a hyphen, an en dash, and em dash.
Moore, B.D., Wallis, I.R., Palá-Paúl, J., Brophy, J.J., Willis, R.H. & Foley, W.J. (2004) Anti-herbivore chemistry of Eucalyptus-cues and deterrents for marsupial folivores. Journal of Chemical Ecology, 30, 1743-1769.

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