Parenthesis: brackets, dashes and commas
English · Key Stage 2 · The School
What parenthesis is
PARENTHESIS is extra information added into a sentence — information the sentence would still make complete sense without. It can be marked off with a pair of commas, a pair of dashes, or brackets. All three do the same basic job, even though they look quite different on the page to a reader's eye.
The removal test
Take out whatever sits between the pair of marks. What is left should still be a complete, working sentence. "My brother, who is older than me, plays football." Remove "who is older than me" — "My brother plays football" still works fine, with nothing missing.
Choosing which mark to use
Commas feel neutral and formal. Dashes feel more informal, and can add emphasis — "the cake — chocolate, obviously — was gone in minutes." Brackets feel like an aside, almost a whisper to the reader — "the trophy (which nobody had cleaned in years) sat on the shelf." Same job, slightly different tone, chosen to match the voice of the piece.
Take one sentence with extra information to add: "The library, which had just been renovated, was busier than usual." Rewrite it with dashes: "The library — which had just been renovated — was busier than usual." Rewrite it with brackets: "The library (which had just been renovated) was busier than usual." Try the removal test on all three: "The library was busier than usual" works every single time. Same information, three different marks, three slightly different tones — but the same underlying rule holds for all of them, whichever one a writer happens to choose for a piece of writing.