Use Em Dashes to Set Off Parenthetical Information
Em dashes are often used to set off parenthetical information. ... For this usage, make sure you use two em dashes. Use one before the parenthetical information and one after it. Putting spaces before and after an em dash is a matter of preference; just be consistent.
Do em dashes have to be in pairs?
The Em Dash
We can use these dashes in pairs, as we would use parentheses—that is, to enclose a word, or a phrase, or a clause (as we've done here)—or they can be used alone to detach one end of a sentence from its main body.
Do you need two dashes?
Two dashes can emphasize material in the middle of a sentence. Some style and grammar guides even permit you to write a complete sentence within the dashes.
