Parentheses (singular: parenthesis) or, outside the U.S., round brackets, set off material that is useful to the reader but less crucial to the meaning of a sentence than information that would be set off by em dashes or commas. Parenthetical words, phrases, and clauses are usually remarks from the writer, informative side-notes, introduced abbreviations, definitions, translations, examples, cross-references to other things within a text, or citations.
The rule of thumb for whether to use parentheses is this: If the meaning of the sentence would be clear without the parenthetical remark, then parentheses are appropriate. When a parenthetical remark contains crucial information, consider an alternative form of punctuation. For example, this sentence would make less sense without the parenthetical text, so the parentheses should be replaced with commas:
Ralph (who had brought his metal detector) spent the day searching for coins on the beach.
Punctuation and parentheses
If the text within the parentheses is syntactically integrated with the surrounding sentence, keep the end punctuation outside the parentheses—for example:
Punctuating parentheses is easy (if you know where to put the period).
But parenthetical information does not always have to be syntactically integrated. For example, a with is implied but omitted at the start of this parenthetical text:
… Trump grew up in a big house (some two dozen rooms, twenty-foot Georgian columns on the portico) in the same borough of New York. [New Yorker]
If the material within parentheses is a complete sentence and does not fall in the middle of a nonparenthetical sentence, consider making the sentences separate. Include the parenthetical text’s end punctuation within the closing parenthesis—for example:
Most of the guests had hamburgers. (The only exception was Ralph.)
But sometimes a complete sentence within parentheses may be enclosed within another sentence. In this case, no end punctuation is needed for the parenthetical text—for example:
Most of the guests (the only exception was Ralph) had hamburgers.
When parenthetical text takes a question mark or exclamation point, put this punctuation within the parentheses—for example:
We saw a small, red bird (a cardinal?) in the tree.
The low price ($10!) made the hamburger combo irresistible.
Parentheses within parentheses
Different publications have different standards for whether to put parentheses within parentheses (like this (and like this)). Some publications use square brackets in place of internal parentheses (like this [and like this]). Some recommend using em-dashes instead of internal parentheses (like this—and like this). In general, it’s best to avoid double-parentheses outside mathematics and law. They put a heavy burden on the reader.