I use em dashes frequently to break up random or ancillary thoughts in my writing. However, I'm uncertain how to deal with punctuation after an em dash when my sentence continues. For example, which of the following is correct?
If at first you don't succeed—I seem to encounter this scenario frequently—try, try again.
If at first you don't succeed—I seem to encounter this scenario frequently—, try, try again.
Would I discard the comma in this case? It seems unnatural to keep it.
My advice would be to avoid this situation if possible, by recasting the sentence. If that is impossible, which I agree it sometimes is, I say you should add the comma. This will give the reader of such an often-complicated sentence the additional help of consistency, ugly though it may be. Fowler, who is usually stingy with punctuation marks, agrees. So does Fowler's modern editor Burchfield, citing him:
... after a second dash 'any stop that would have been used if the ... dashes and their contents had not been there should still be used'.
According to the Chicago Manual of Style, you should drop the comma. See section 6.86: "In modern usage, 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." No word on commas that separate independent clauses from each other, however, which is the case that brought me here.
Luckily, Wilson Follett's Modern American Usage contains a relevant example sentence: "C. had questioned everyone who had seen the shooting—but did he know for certain who owned the gun?"
