Paired Dashes and Parentheses

A single dash signals a break and lets the sentence lean on what comes after it. Two dashes do something different: they act like a pair of handles that pick up a chunk of extra information and set it off from the main clause, one dash to open the interruption and one to close it. Parentheses work the same way, just quieter. Both devices only function as a matched set. If you open with one, you must close with the other, and the words inside are always optional extras, never load-bearing parts of the sentence.

Because that inserted chunk is optional, the test for correctness is simple: delete everything between the two marks and read what remains. If the leftover sentence still has a subject, a verb, and a complete thought, the bracketing works. If the subject or verb was hiding inside the brackets, the sentence collapses when you remove them, which means the punctuation is wrong. This is why 'The comet - last seen in 1986 - will return in 2061' works: strip the dashes and their contents, and 'The comet will return in 2061' still stands.

This sits next to the single dash and the colon in the same family of internal punctuation, but it answers a different question. The colon and single dash introduce or launch something forward; paired dashes and parentheses interrupt and then return you to the original sentence. Choosing between a dash pair and a parenthesis pair is mostly a question of emphasis, not correctness, which is where this topic connects to tone and style rather than grammar rules.