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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?

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    I think it's fine the way it is.
    – Hot Licks
    Mar 16 at 22:59
  • You can rephrase the sentence to begin with "We need to have John in the game when rebounding becomes important – as it was when playing the Lakers." This would make it easier to extend the subordinate clause. Mar 16 at 23:02
  • I believe the question was about en-dashes not em-dashes, which is what I thought I wrote. Did someone edit my question or did I just make a mistake?
    – Smithey
    Mar 19 at 21:52
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    It appears you did originally ask about en dashes, but in this case the mark in question is in fact the em dash. Em dashes (—) are typically used to set apart phrases/clauses, as is the case in this sentence; en dashes (–) are primarily used to indicate ranges. It does so happen that in British English spaced en dashes are frequently used in lieu of unspaced em dashes, so perhaps that is what you were going for here; but your use of the double hyphen is normally indicative of an em dash, as en dashes are shorter than that.
    – GrammarCop
    Mar 19 at 22:18
  • There is no absolute requirement to demarcate the main clause from a subordinate one using a comma. I tend to do so but I've read many scholarly books written by Brits where the comma is rarely used in that manner. It is a matter of style.
    – TimR
    Mar 20 at 12:10

2 Answers 2

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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.

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  • Ok thanks, but I just want to make sure it's clear when the subordinate clause ends. I'm afraid the reader could get lost if the phrase between the dash is too long. You're right the comma (if there were one, which you say there shouldn't be) should be after the second dash.
    – Smithey
    Mar 19 at 21:55
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    I think that in most cases context should allow the reader to extract the proper meaning. But I’m sure there are instances in which it would be ambiguous; in such a case, it would be best to rewrite the sentence in a way that makes it clearer—perhaps by moving the subordinate clause to follow the independent clause. Either way, it’s still poor style to have a dash and comma follow each other directly.
    – GrammarCop
    Mar 19 at 22:09
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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.

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  • Ok but I think I'm asking about en-dashes not em-dash. I thought I wrote that but maybe I mixed it up.
    – Smithey
    Mar 19 at 21:56
  • 2
    @Smithey No, parentheticals go between em dashes, not en dashes.
    – Lambie
    Mar 19 at 22:19
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    You did write “en dash” but you showed em dashes (“--” the double dash). En dashes are irrelevant here. Mar 20 at 0:47
  • @TinfoilHat I see, thank you.
    – Smithey
    Mar 22 at 5:47

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