So as not to mislead anyone, the parent is mostly incorrect:
Here's an example sentence: Semicolons must have independent clauses—phrases that could form a full sentence on their own—on both sides of them; they are essentially alternatives for periods. Em dashes don't require independent clauses on either side.
In the italicized sentence,
* phrases that could form a full sentence on their own is not an independent clause but is valid between em dashes. on both sides of them, after the em dashes, is also not an independent clause. (The em dashes function like commas or parentheses here.)
* The parts before and after the semicolon are independent clauses. You could replace the semicolon with a period and you'd have perfectly valid grammar. I just chose to connect the two sentences a bit more.
I don't know if you can use em dashes as the parent comment describes, connecting three independent clauses:
* My favorite fruit is peaches—they are very sweet—I eat them all summer.
I think the above is wrong; it should be one of the following:
* My favorite fruit is peaches—they are very sweet—and I eat them all summer.: The last section is a dependent clause made by "and", not an independent clause.
* My favorite fruit is peaches—they are very sweet; I eat them all summer.: One both sides of the semicolon are independent clauses; I could replace the semicolon with a period.
Maybe there are examples I'm not thinking of? I infer that the rule might be that the punctution following the em-dashed clauses should be the punctuation that would have been used without the em-dashed clause, but that's based on very limited evidence.
Here's an example sentence: Semicolons must have independent clauses—phrases that could form a full sentence on their own—on both sides of them; they are essentially alternatives for periods. Em dashes don't require independent clauses on either side.
In the italicized sentence,
* phrases that could form a full sentence on their own is not an independent clause but is valid between em dashes. on both sides of them, after the em dashes, is also not an independent clause. (The em dashes function like commas or parentheses here.)
* The parts before and after the semicolon are independent clauses. You could replace the semicolon with a period and you'd have perfectly valid grammar. I just chose to connect the two sentences a bit more.
I don't know if you can use em dashes as the parent comment describes, connecting three independent clauses:
* My favorite fruit is peaches—they are very sweet—I eat them all summer.
I think the above is wrong; it should be one of the following:
* My favorite fruit is peaches—they are very sweet—and I eat them all summer.: The last section is a dependent clause made by "and", not an independent clause.
* My favorite fruit is peaches—they are very sweet; I eat them all summer.: One both sides of the semicolon are independent clauses; I could replace the semicolon with a period.
Maybe there are examples I'm not thinking of? I infer that the rule might be that the punctution following the em-dashed clauses should be the punctuation that would have been used without the em-dashed clause, but that's based on very limited evidence.