Why is it acceptable sometimes for writers to enclose an independent clause--a clause which could stand by itself as its own sentence--in dashes and parentheses, yet this can't be done with commas? An example sentence:

  1. "She won the raffle--this had never happened to her before."
  2. She had won the raffle (this had never happened to her before).
  3. She had won the raffle, this had never happened to her before.

Sentence 3. is clearly a splice, yet the comma is functioning in the same way as the dash is in sentence 1. (I apologise for the poor example.)

On a slightly separate note, is the em dash ever functioning in the same way as the colon? I know that in sentence 1., above, a colon could replace the dash, and therfore they are interchangeable, but does the dash always only serve as a parenthetical piece of punctuation or can it function like the colon?

  • The em dash is used for parenthetical statements and for disjuncts. If you have ever thought that it can function like a colon, it is probable that you confused what you were seeing with a disjunct. Example: "So that is my comment on football—what were we talking about?" – Robusto Aug 19 '15 at 14:04
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    A colon could not replace the em dash or fix the comma-splice. It would need to be a semi-colon. – Carl Smith Aug 19 '15 at 14:05
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    If you've read somewhere that 'parentheticals can be set off by parentheses, dashes or commas', (1) not all parentheticals can; (2) one choice may be better than another even when they are both available; (3) sometimes even zero punctuation is an option; (4) this example is perhaps better regarded as two related sentences, requiring a semicolon at least. – Edwin Ashworth Aug 19 '15 at 14:07
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    @CarlSmith A colon is acceptable here. Its function is extremely loosely defined; I've even seen 'to introduce something further'. In this case, though it's a rather contrived example, preceding context could easily make it a good choice. – Edwin Ashworth Aug 19 '15 at 14:09
  • I always thought a colon was only for setting something up, like Consider this: or Answer the following questions: or The Ten Commandments are:. – Carl Smith Aug 19 '15 at 14:36

If you can replace one punctuation mark with another, this doesn't mean they are interchangeable.

She won the raffle. This had never happened to her before.

She won the raffle; this had never happened to her before.

They're different because the first variant is two sentences and the second is a single sentence. But they're both correct.

She won the raffle – this had never happened to her before.

Correct, but it doesn't make ";" and "–" interchangeable generally; "–" is used in dialog to jam incomplete sentences together or indicate pauses, as in "She said – oh, what was it? She said – I'm trying to remember – no, that's not it – I forget what she was saying, but – oh, yes. She said, 'Your sentences are disjointed messes.'"

She won the raffle, this had never happened to her before.

Not correct, as noted in the question.

There's often more than one way to say something, and more than one way to punctuate it. That's language for you.

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