How do you punctuate after a terminal em dash?

A particular use of punctuation jumped out at me as I read a recently published book.

In general, I observe punctuation rules, so I’d like to know if the one used in this book follows an old rule, a new rule, or a house style. I question whether it’s a house style though–unless those change depending on the author—because I have books published by the same house in which this particular style is not used.

EM DASH — This author, as many of us do, used an em dash to indicate interrupted speech. But what struck me was a difference in the punctuation used after that dash. For instance, I would write such a sentence this way:

“I couldn’t possibly let you—”

“Let me? Let me!” His outrage jerked him to his feet. “Since when do I require your permission?”

But if those lines appeared in this book, the first would have been punctuated this way:

“I couldn’t possibly let you—.”

“Let me? Let me!” His outrage jerked him to his feet. “Since when do I require your permission?”

You’ll notice the period after the em dash. I’ve since pulled novels off my shelves searching for sentences with interrupted speech. I haven’t found one yet that duplicates this author/editor’s construction.

Am I behind the times or have I, and countless other authors, been doing it wrong all along?

Your turn: Please tell me, how do you punctuate after a terminal em dash?

(See my next post on using punctuation with the terminal ellipsis.)

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