Brackets, or crotchets, are always used in pairs to mark off material inserted into a quotation which is not part of the original quotation. The use of brackets should be limited, but may include short references, short definitions, a short piece of information which clarifies the quotation, or an editorial comment.
The Latin word sic, which means “thus” or “so,” is often put into brackets to indicate a misspelling or some other misuse of language in the original quotation.
Brackets are also used in dictionaries, glossaries, and word lists to show word origins and etymologies.
Brackets may be used to show parenthetical information for material already inside parentheses.
Then Ceres asked: Tell me, heavenly bow,
If Venus or her son [Cupid], as thou dost know,
Do now attend the queen.
(Clarifies the meaning)
Misspelling in original quotation:
“Mi dere Jo I hope u r write [sic] well.”
Word origin: Brackets [L.]
(The word brackets comes from Latin.)
Parentheses within parentheses: (Charles Dickens [1812-1870] had been trained as a stenographer.)