Comma with Em-dash after Subordinate Clause [duplicate]

Question

Consider the sentence:

When rebounding becomes important -- as is the case when playing the Lakers -- we need to have John in the game.

Is there a need for a comma after "important" to mark the subordinate clause, which is a separate matter from the tangential phrase between dashes?

In this case, the sentence seems fine the way it is, but if the clause between dashes was longer, it might get harder to see where we're going.

There are prior questions asking this, but none of them have a clear answer. Someone in the comments said that the em-dash does replace the subordinating comma, but I'd like a proper answer and a rationale please. Thank you.

Does the em-dash replace a subordinating comma?

Answer 1

No, it is correct without a comma. If the visually displeasing nature of

When rebounding becomes important,—as is the case when playing the Lakers—we need to have John in the game.

isn’t good enough a reason for you, a dash functionally takes the place of a comma (or colon/semicolon/parentheses depending on context). You cannot have both a comma and a dash directly next to each other, as is the case for all other forms of sentence punctuation barring parentheses. Think of it this way: If you were to set apart the parenthetical with commas, which would be just as correct syntactically, would you include a double comma? In other words, would you write this?

When rebounding becomes important,, as is the case when playing the Lakers, we need to have John in the game.

Certainly not! You would just use a single comma performing both functions. So why would the rule be any different for a dash? In many cases punctuation marks are subsumed by others. Take dialogue as an example; when a quotation ends with a question mark, that mark also ends the whole sentence, as in

John asked, “Where are you going?” (Notice the lack of a period after the quote.)

You say the subordinate clause “is a separate matter from the tangential phrase between dashes,” but I disagree. The parenthetical is directly related to the subordinate clause; it describes a certain instance of when rebounding is important. If you were to add a comma somewhere, it would be best placed after the second dash. But once again, it is best left out.

Answer 2

No, there should not be a comma in front of either em dash in:

When rebounding becomes important—as is the case when playing the Lakers—we need to have John in the game.

According to The Chicago Manual of Style (emphasis added):

In modern usage, a question mark or an exclamation point—but never a comma, a colon, or a semicolon—may precede an em dash. A period may precede an em dash if it is part of an abbreviation. . . .

If the context calls for an em dash where a comma would ordinarily separate a dependent clause from an independent clause, the comma is omitted. Likewise, if an em dash is used at the end of quoted material to indicate an interruption, the comma can be safely omitted before the words that identify the speaker. . . .

_{Because the data had not been fully analyzed—let alone collated—the publication of the report was delayed.}

_{“I assure you, we shall never—” Sylvia began, but Mark cut her short.}