Use brackets to enclose parenthetical elements in material that is itself within parentheses and to enclose explanatory words or comments that you are inserting into a quotation.

Setting off material within parentheses

Eventually the investigation had to examine the major agencies (including the previously sacrosanct National Security Agency [NSA]) that were conducting covert operations.

Inserting material within quotations

Massing notes that “on average, it [Fox News] attracts more than eight million people daily—more than double the number who watch CNN.”

The bracketed words clarify it in the original quotation.

In the quotation in the following sentence, the artist Gauguin’s name is misspelled. The bracketed word sic, which means “so,” tells readers that the person being quoted—not the writer who has picked up the quotation—made the mistake.

One admirer wrote, “She was the most striking woman I’d ever seen—a sort of wonderful combination of Mia Farrow and one of Gaugin’s [sic] Polynesian nymphs.”