Parentheses and brackets must never be used interchangeably.
Rule 1. Use parentheses to enclose information that clarifies or is used as an aside.
Example: He finally answered (after taking five minutes to think) that he did not understand the question.
If material in parentheses ends a sentence, the period goes after the parentheses.
Example: He gave me a nice bonus ($500).
Commas could have been used in the first example; a colon could have been used in the second example. The use of parentheses indicates that the writer considered the information less important—almost an afterthought.
Rule 2a. Periods go inside parentheses only if an entire sentence is inside the parentheses.
Example: Please read the analysis. (You'll be amazed.)
This is a rule with a lot of wiggle room. An entire sentence in parentheses is often acceptable without an enclosed period:
Example: Please read the analysis (you'll be amazed).
Rule 2b. Take care to punctuate correctly when punctuation is required both inside and outside parentheses.
Example: You are late (aren't you?).
Note the question mark within the parentheses. The period after the parentheses is necessary to bring the entire sentence to a close.
Rule 3. Parentheses, despite appearances, are not part of the subject.
Example: Joe (and his trusty mutt) was always welcome.
If this seems awkward, try rewriting the sentence:
Example: Joe (accompanied by his trusty mutt) was always welcome.
Rule 4. Commas are more likely to follow parentheses than precede them.
Incorrect: When he got home, (it was already dark outside) he fixed dinner.
Correct: When he got home (it was already dark outside), he fixed dinner.
Brackets are far less common than parentheses, and they are only used in special cases. Brackets (like single quotation marks) are used exclusively within quoted material.
Rule 1. Brackets are interruptions. When we see them, we know they've been added by someone else. They are used to explain or comment on the quotation.
"Four score and seven [today we'd say eighty-seven] years ago..."
"Bill shook hands with [his son] Al."
Rule 2. When quoting something that has a spelling or grammar mistake or presents material in a confusing way, insert the term sic in italics and enclose it in nonitalic (unless the surrounding text is italic) brackets.
Sic ("thus" in Latin) is shorthand for, "This is exactly what the original material says."
Example: She wrote, "I would rather die then [sic] be seen wearing the same outfit as my sister."
The [sic] indicates that then was mistakenly used instead of than.
Rule 3. In formal writing, brackets are often used to maintain the integrity of both a quotation and the sentences others use it in.
Example: "[T]he better angels of our nature" gave a powerful ending to Lincoln's first inaugural address.
Lincoln's memorable phrase came midsentence, so the word the was not originally capitalized.