The word “brackets” can refer to either the square variety [like this] or to parentheses (like this). Some sticklers think that only square brackets deserve the name, but there’s no need to be so restrictive. Here we review the main uses of square brackets.
Square brackets are used to insert extra words and explanations in a quotation:
The novel Post Captain opens with Captain Jack Aubrey looking both heroic and uncertain of his own future in the navy: “Captain Aubrey was standing by the aftermost larboard carronade [the last cannon on the left side of the ship], with a completely abstracted, non-committal look upon his face.”
The bank sent them a letter stating, “This letter verifies that the customer [me] has an account with us.”
Things get complicated when you need to change the quotation to integrate it properly. Sometimes it’s necessary to alter the capitalization of a word. Let’s say you want to quote the following passage:
“As art historians, we often miss the point that Kitsch’s paintings fetched so much money in post–World War I Germany because inflation was rampant.”
However, if you chop off the first part of the sentence, your quote will look like this:
As Edward Jones observes, “[W]e often miss the point that Kitsch’s paintings fetched so much money in post–World War I Germany because inflation was rampant.”
The reason is that if your signal phrase (your own words) ends with a comma, and the quotation is a complete sentence, the latter needs to be capitalized. You could avoid using square brackets if you change your signal phrase:
Edward Jones observes that “we often …”
This time the signal phrase and the quote form a complete sentence together, which is why capitalization is unnecessary.
Some writers go all out—changing pronouns and adding words—but square brackets should be used as little as possible. If you’re using two or more sets of brackets, there is probably a more effective way of integrating the quotation.
Sometimes when you’re quoting a passage the author has made an error—most often a simple spelling mistake. To indicate that you’ve transcribed the quotation faithfully, and that this is not your error, you can add sic in square brackets. In Latin, sic means so or thus, and is short for sic erat scriptum (it was written thus).
In the following example, the writer points out that in the quotation the name of the yacht is written incorrectly (it should be Granma):
Daniel Martin writes, “A pivotal moment in the mythical life of Che Guevara occurred when he sailed to Cuba in the yacht Grandma [sic]” (3).
Notice that sic does not have to be in italics.
While it’s satisfying to point out an error, adding sic can seem rather pretentious, and should be kept to a minimum.
If you’re using brackets inside parentheses, you can make them square:
Dear parenthesis, I am an angular square bracket with a passion for punctuation. If you share my feelings, please get in touch (my number is  286-9901).
This is not a hard and fast rule though. For instance, in the sciences, some formulas that use parentheses may need to go inside square brackets. An example would be algebra equations [(a + 3) ÷ 2 = 6].