When should I use square brackets?

In British English, [ ] = square brackets
In American English, [ ] = brackets
A bracket is a punctuation mark used in matched pairs within text, to set apart or interject other text. Why would we want to interject text into other text? There are a variety of reasons, including:

to add missing words:
  • ”Whether or not she acted competently is [a] fair question to ask.”
to add editorial or authorial comment
  • The political aide will not be testifying [my emphasis].
to add clarification
  • The witness said: "He [the congressman] told me to ignore the subpoena."
to add information
  • Top Secret information was on a variety of computers. [the congressman’s computer, his aide’s computer, his aide’s ex-husband’s computer, etc.].
to modify a direct quotation:
  • She "love[s] lying." (The original words were "I love lying.")
for nesting
  • Square brackets can also be nested (using square brackets [like these] inside round brackets).
Purdue University Grammar Lab

Sentences demonstrating the use of square brackets

  • “Books used [in classes] show solved problems but do not show how to solve problems.”
  • Ron did not agree with the report blaming him [the congresswoman] for the loss.
  • The teacher wrote, "I am annoyed by the increase in the use of [square] brackets on your website.”
  • He continued, “Is there a reason for this [phenomenon]?”
  • ”The overuse of square brackets seems to be most prevalent in your use of [reported] speech.“
  • "I was in awe of the team; [Eric] Cantona, [David] Beckham, [Andrei] Kanchelsis, and the others."
  • ”When I am interviewed by a newspaper columnist, how will I know what the interviewer [or editor] will insert into my comments?"
  • The student proclaimed, "Perhaps somebody will explain the reason for all of them [the square brackets]."
  • He blamed public school teachers for the growing [mis]use of square brackets.
  • The man ordered two pints of Kronenbourg [lager] and a packet of [sea] salt and [balsamic] vinegar crisps.
  • His wife ordered a glass of [dry] white wine and a [Cornish] pasty.

Example sentences using square brackets

  • “Books used [in the seminar] show solved problems but do not show how to solve problems.”
  • ”The misuse of square brackets is most prevalent in your use of [reported] speech.“
  • His wife ordered a glass of [almond] milk and a [Danish] pastry.
University of Illinois Grammar Guide

Other articles you may find helpful

Should I use 'laying' or 'lying'?

How do I use linking verbs?