Matching Both Sides of a Punctuation Pair

Think of the two marks around an interrupter as a matched set, like opening and closing brackets. The mark itself doesn't matter much — commas, dashes, and parentheses can all set off an aside — but whichever one you pick has to show up on both ends. 'The car, a rusted sedan - refused to start' breaks because the comma after 'car' promises a comma later, and instead the sentence delivers a dash. The reader's eye builds one kind of frame and gets handed another.

The mechanism is simple: find where the interrupter starts, find where it ends, and check that the opening mark equals the closing mark. 'Her brother - a paramedic, worked the night shift' has a dash opening the aside and a comma closing it. Since the aside begins with a dash, it must end with a dash: 'Her brother - a paramedic - worked the night shift.' You could also rewrite both sides as commas; either fix works, as long as the two marks match.

This matters because a stronger mark isn't an upgrade. A dash is more forceful than a comma, but swapping one in on just one side doesn't improve the sentence — it just creates a second kind of error, an unmatched pair, on top of whatever was already wrong. This topic sits next to the broader punctuation-pair rules: once you've confirmed both marks match, you still check that the interrupter itself is truly removable, and that no mark inside it is doing a clause's job instead.