em dash: spacing and capitalization
Spaces around an em dash and capitalization after it.
Write an em dash without spaces on either side: The spare—the only one—was gone. The same spacing applies beside quotation marks: “The address”—she checked her phone—“is correct.”
After an em dash, continue in lowercase, even when a complete clause follows: We waited—nothing happened. Keep capitals that a word requires for its own sake: She objected—I agreed. Only one applicant remained—Maria.
Capitalize the first word of a complete quoted sentence introduced by a dash: His reply was brief—“We cannot help you.”
Note. Some publications put spaces around em dashes. Our usage dictionary recommends closed dashes, written without spaces.
Examples
Use The payment—already overdue—arrived today. (No spaces around either dash.)
Use We searched the drawer—the receipt was missing. (A complete clause after a dash continues in lowercase.)
Use One agency responded—NASA. (The acronym keeps its capitals.)
Avoid The road was blocked—We turned back. (Do not capitalize we merely because it follows a dash.)