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Square brackets, often simply called brackets, are more disconnective than parentheses. They are used to enclose material too extraneous for parentheses.
Use brackets for editorial comments or additional information on material written by someone else. (To use ordinary parentheses for this purpose would give the impression that the inserted words were those of the person quoted.)
- The witness, who changed his story on the stand, issued the following statement to reporters, “I only backed up the defendant’s story because he threatened me [my emphasis].”
Square brackets should also enclose translations given immediately after short quotations, terms and titles of books or articles.
- Canada’s motto A Mari usque ad Mare [“from sea to sea”] is taken from Psalm 72, verse 8.
Replacing parentheses within parentheses
Parentheses within parentheses should be used sparingly, except in legal and scholarly texts and specifically for letters and numerals referring to subsections of a document:
- (See section 14(i)(c))
Therefore, when one set of parentheses is to be placed within another, replace the inner parentheses with square brackets.
- Acadia (from Algatem [“dwelling here and there”])
- He worked hard (twelve hours a day [and no bonus for overtime], seven days a week) until the task was completed.
As an alternative, one set of parentheses may in some cases be replaced by dashes:
- He worked hard—twelve hours a day (and no bonus for overtime), seven days a week—until the task was completed.
Replacing parentheses in a series
Square brackets may also be used in place of round brackets where two or more sets of the latter would otherwise occur in succession:
- in 3(a) [according to Bixby’s enumeration]…
- Here, f(x) [c. s. 8.3] reaches a maximum when…
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