Matched-pair rule
SAT glossary · Reading & Writing
Definition
Punctuation that walls off non-essential material must open and close with the same mark: dash–dash or comma–comma, never mixed. It is a favorite item because the two marks look so different that students do not think of them as members of one system.
Matched-pair rule in plain English
This is a punctuation rule about matching: any punctuation marks used to 'wall off' a chunk of non-essential material from the rest of the sentence have to be a MATCHING PAIR — either two dashes, or two commas — and you're never allowed to mix them (like starting with a dash and closing with a comma). This shows up as a favorite trap specifically because a dash and a comma look so visually different that people don't automatically think of them as being part of the exact same punctuation 'system' with the exact same job.
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