Brackets

1. Enclose words or passages in quoted matter to indicate insertion of material written by someone other than the author: A tough but nervous, tenacious but restless race [the Yankees]; materially ambitious, yet prone to introspection. …—Samuel Eliot Morison
2. Enclose material inserted within matter already in parentheses: (Vancouver [B.C.] January 1, 19—).

See also: Parentheses and Brackets.