Brackets allow the insertion of editorial material inside quotations. They can be used for the following purposes:
If the original material includes a noun or pronoun that is unclear, brackets can be used for clarification.
The president stated that he “will not sign the bill they [Republican members of the House] have been talking about.”
In his memoirs, the author reveals, “The year we moved into the house  was a difficult one for us, both emotionally and financially.”
The media mogul was overheard saying, “I would never do a deal with [Acme Corporation's CEO,] Wile E. Coyote.”
When used in this way, the bracketed information should be an addition, not a substitution. For example, if the original quotation is “She never called back,” do not change it to “[Lucy] never called back.” Instead write: “She [Lucy] never called back.” (Note: Many newspapers ignore this rule. In professional and academic writing, it is better to follow it.)
In many cases, brackets can be avoided by reframing the quotation.
Awkward: “Why can’t we do the same thing [provide government-funded grants to independent filmmakers] in this country?” Christina Black asks.
Recast: Citing filmmaking grants provided by the Australian government, independent filmmaker Christina Black asks, “Why can’t we do the same thing in this country?”
If a quotation includes a foreign word or phrase that might not be understood, provide a translation in brackets. (Use parentheses for translations of unquoted material.)
Smith writes in his autobiography: “I seldom spoke in French class. When I did, I usually just said je ne sais pas [I don’t know].”
In most contexts, it is acceptable to silently change the first letter of quoted material from uppercase to lowercase, or vice versa. In certain contexts, such changes must be indicated with brackets.
“[T]his study has been widely cited, notwithstanding its dubious methodology.”
Under the terms of his employment contract, his “[p]erformance-based stock options shall not vest until December 31, 2015.”
The Latin term sic, meaning “so” or “thus,” is used to indicate an error or confirm an unusual usage in the original material. Without the sic, a reader might wonder if the error was made by the writer offering the quotation. Note that sic should be italicized, but the brackets containing it should not.
The final report indicated that “pilot error were [sic] the most likely cause of the crash.”
As an alternative, reframe the quotation to eliminate the error.
“Pilot error,” according to the final report, was “the most likely cause of the crash.”
If you suspect, but are not certain of, an error in the original material, a bracketed guess and question mark is appropriate.
“The architect appears to have been heavily influenced by the Bacchus [Bauhaus?] style.”
If you use italics to emphasize a portion of the quotation, indicate the change in brackets.
She said she would consider “a very short extension of the deadline, but only under the most extraordinary circumstances [emphasis added].”
An alternative approach is to note the emphasis outside the quotation, in parentheses, either as a separate sentence immediately after the sentence containing the quotation:
She said she would consider “a very short extension of the deadline, but only under the most extraordinary circumstances.” (Emphasis added.)
or as a parenthetical note added to the end of the sentence containing the quotation:
She said she would consider “a very short extension of the deadline, but only under the most extraordinary circumstances” (emphasis added).
If the original material contains language you deem inappropriate for your audience, brackets can be used to remove it.
He told them to “sit the [expletive] down.”
In the rare event that parentheses are required within parentheses, use brackets instead. This is one of the few uses of brackets outside of quotations.
Correct: In his twenties, he toured the country giving lectures to physics students (subsequently published as M-theory for Morons ).
Incorrect: In his twenties, he toured the country giving lectures to physics students (subsequently published as M-theory for Morons (2008)).
If the material being quoted already contains brackets, this should be noted.
Richardson finds support for his position in an earlier study by the Somesuch Foundation: “The authors acknowledge that ‘during the four years he [George Clinton] was president, average real wages were flat.’” (Brackets in original.)