How to Use Parentheses in Writing

The parenthesis is a punctuation mark, which is written or typed as an upright curved line. Two parentheses, ( ), are generally paired and used to mark off explanatory or qualifying remarks in writing. Parentheses indicate an interrupting phrase, a word group (a statement, question, or exclamation) that interrupts the flow of a sentence and can also be set off with commas or dashes.

The parenthesis is a type of bracket, which when paired with another bracket—[ ]—is used to interject text within other text. Parentheses are prevalent in mathematics, too, where they are used to set off arithmetic symbols as well as numbers, operations, and equations. 

Origins of the Parenthesis

The symbols themselves first showed up in the late 14th century, with scribes using virgulae convexae (also called half moons) for a variety of purposes. By the end of the 16th century, the parenthesis (from the Latin for "insert beside") had begun to assume its modern role, as Richard Mulcaster explained in "Elementarie," which was published in 1582:

"Parenthesis is expressed by two half circles, which in writing enclose some perfit branch, as not mere impertinent, so not fullie concident to the sentence, which it breaketh, and in reading warneth us, that the words inclosed by them ar to be pronounced with a lower & quikker voice, then the words either before them or after them."

In her book "Quoting Speech in Early English," Colette Moore notes that parentheses, like other marks of punctuation, originally had both "elocutionary and grammatical" functions:

"[W]e see that whether through vocal or syntactic means, the parentheses are taken as a means to downplay the significance of the material enclosed within."

Spanning more than 400 years (Moore's book was published in 2011), both authors say essentially the same thing: Parentheses separate text that, while important in that it adds meaning, is less significant than the text that falls outside of these punctuation marks.

Purpose

Parentheses allow for the insertion of some verbal unit that interrupts the normal syntactic flow of the sentence. These are called parenthetical elements, which may also be set off by dashes. An example of parentheses in use would be:

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"The students (it must be acknowledged) are a foul-mouthed bunch."

The important information in this sentence is that the students are foul-mouthed. The aside adds texture to the sentence, but the statement would work fine and make sense without the parenthetical information. The Chicago Manual of Style Online explains that parentheses, which are stronger than commas or dashes, set off material from the surrounding text, adding that; "Like dashes but unlike commas, parentheses can set off text that has no grammatical relationship to the rest of the sentence." The style guide gives these examples:

• Intelligence tests (e.g., the Stanford-Binet) are no longer widely used.
• Our final sample (collected under difficult conditions) contained an impurity.
• Wexford’s analysis (see chapter 3) is more to the point.
• The disagreement between Johns and Evans (its origins have been discussed elsewhere) ultimately destroyed the organization.

The style manual also notes that you can use parentheses as delimiters for letters or numbers in a list or outline, as well as in academic uses including parenthetical references to a list of works cited.

Using Parentheses Correctly

Parentheses (as with other punctuation marks) can be tricky to use until you understand a few simple rules:

Adding additional information: June Casagrande, author of "The Best Punctuation Book, Period.", notes that you can use parentheses to convey additional information, such as:

• The new sedan is fast (it goes from zero to 60 in just six seconds).
• The boss (who had walked in just in time to see the accident) was furious.
• She strolled the third arrondissement (district).

In the first sentence, the statement, The new sedan is fast, does not end with a period. Instead, you place the period after the parenthetical sentence (as well as the final parenthesis), it goes from zero to 60 in just six seconds. You also start the parenthetical sentence with a lowercase letter (i) because it is still considered part of the overall sentence and not a separate statement.

In the second sentence, you might argue that the parenthetical information (the fact that the boss saw an accident) is key to understanding the sentence. In the third sentence, the parenthetical word district is an English translation of the French word arrondissement. Though the word district is parenthetical, it might be important in helping a non-French-speaking reader understand the sentence.

Delimiters for letters or numbers in a list: The Chicago Manual of Style says you should put parentheses around each number or letter in a list, as in these examples:

• Compose three sentences to illustrate analogous uses of (1) commas, (2) em dashes, and (3) parentheses.
• For the duration of the experiment, the dieters were instructed to avoid (a) meat, (b) bottled drinks, (c) packaged foods, and (d) nicotine.

In-text citations/reference information: The Chicago Manual calls them parenthetical citations, while the American Psychological Association (which sets APA style) calls them in-text citations. These are citations placed within the text in an academic paper, journal article, or book that points the reader to a more complete citation in the bibliography or references section. Examples, as noted by Purdue OWL, are:

• According to Jones (2018), "Students often had difficulty using APA style, especially when it was their first time" (p. 199). 
• Jones (2018) found "students often had difficulty using APA style" (p. 199); what implications does this have for teachers?
• The study participants showed no improvement in cholesterol levels (McLellan and Frost, 2012).

For these types of parenthetical citations, you generally include the year of the publication, the author(s)' names, and, if needed, the page number(s). Note also that in the previous sentence, you can use parentheses around a single letter, indicating that the word "number" may be singular referring to a single page number, or it may be plural, referring to two or more page numbers or that there may be only a single author or several authors.

Mathematical problems: In math, parentheses are used to group numbers or variables, or both. When you see a math problem containing parentheses, you need to use the order of operations to solve it. Take as an example the problem: 9 - 5 ÷ (8 - 3) x 2 + 6. In this problem, you would calculate the operation within the parentheses first, even if it is an operation that would normally come after the other operations in the problem.

Parenthetical Observations

Neil Gaiman really likes parentheses. Biographer Hank Wagner quoted the British author in "Prince of Stories: The Many Worlds of Neil Gaiman" explaining why he is a fan of these curved punctuation marks:

"I admired [C.S. Lewis's] use of parenthetical statements to the reader, where he would just go talk to you. Suddenly the author would address a private aside to you, the reader. It was just you and him. I'd think, 'Oh, my gosh, that is so cool! I want to do that! When I become an author, I want to be able to do things in parentheses.' "

Gaimen may feel blessed when the author offers him a "personal" aside, but other writers say that parentheses may be a clue that the sentence is becoming contorted. As author Sarah Vowell notes in her book, "Take the Cannoli: Stories From the New World," with a touch of sarcasm:

"I have a similar affection for the parenthesis (but I always take most of my parentheses out, so as not to call undue attention to the glaring fact that I cannot think in complete sentences, that I think only in short  fragments or long,  run-on thought relays that the literati call  stream of consciousness but I still like to think of as disdain for the finality of the period)."

So take the advice of "The Associated Press Stylebook." Be kind to your readers and use parentheses sparingly. Rewrite your sentence if you find you are including long asides or more than one set of parentheses. Use these punctuation marks only when you have a short, pithy, and interesting bit to convey to readers to heighten their interest—not confuse them.