You’ve probably seen this guy before. It looks a lengthened hyphen, and it’s named "em dash" because it takes up the same amount of room as a capital letter "M."
More useful than a hyphen, this dash is the workhorse of your punctuation toolkit. You can use the em dash to replace commas, parentheses, or colons, but the meaning changes slightly in each situation.
Em Dash Replacing Commas
Using a pair of em dashes to replace commas in a sentence can help improve readability. However, the effect will be more forceful. Use this when you want to emphasize the words inside the em dashes.
When Julie turned in her term paper — two weeks after the due date — her professor told her she would automatically receive two letter grades off her final grade.
Em Dash Replacing Parentheses
A pair of em dashes can also replace a pair of parentheses. When writing, consider the point you are trying to make. Dashes are less formal, but they also make the content stand out. Use dashes to call attention to the material and parentheses to be more subtle. Other punctuation that would be used with the parentheses, like commas, should be removed.
Julie had a lot of excuses for her professor (five emails' worth), but he wouldn’t budge on the grade.
Julie had a lot of excuses for her professor — five emails' worth — but he wouldn’t budge on the grade.
When an em dash is used in place of parentheses at the end of a sentence, only a single dash should be used.
Julie finally accepted the grade she deserved (but knew she would have to make it up on the final exam).
Julie finally accepted the grade she deserved — but knew she would have to make it up on the final exam.
Em Dash Replacing a Colon
Use an em dash at the end of your sentence as a way to emphasize the conclusion in an informal manner.
Julie made sure to attend every study session offered by the professor and was rewarded with her ultimate goal — a passing grade.
How to Use the Em Dash
This punctuation mark is only coming to play in written text — you’re probably not speaking the em dash like a hashtag. Modern word processors make it easy, and most will create the em dash when you type a pair of hyphens. It’s also up to you whether you want to insert spaces around the em dash. AP style — which is followed by most news organizations — is to insert a space before and after the em dash. If you choose not to — or your organization has its own style guide — that’s perfectly correct as well.