Parentheses, Brackets, and Braces: When to Use Each

Overview: The Three Types of Brackets

English uses several types of bracketing punctuation marks, each with distinct purposes. While they all enclose information, choosing the wrong type can confuse readers or violate established conventions.

In everyday writing, parentheses are by far the most common. Square brackets appear primarily in academic and journalistic writing. Curly braces and angle brackets are specialized tools used mainly in mathematics, computer science, and linguistics.

Parentheses ( ): Rules and Uses

Parentheses—also called "round brackets" in British English—are the most versatile and frequently used brackets in English writing. They enclose supplementary information that clarifies, elaborates, or adds an aside without disrupting the main flow of the sentence.

Adding Supplementary Information

The most common use of parentheses is to add information that is helpful but not essential to the sentence:

• The Eiffel Tower (completed in 1889) is the most visited paid monument in the world.
• The CEO (who has been with the company since its founding) announced her retirement.
• The experiment yielded positive results (see Table 3).

Defining Abbreviations and Acronyms

When introducing an abbreviation or acronym, parentheses enclose either the full term or the short form:

• The World Health Organization (WHO) issued new guidelines.
• The search engine optimization (SEO) strategy was effective.

Enclosing Numbers or Letters in Lists

• Please bring (1) your passport, (2) a pen, and (3) the completed form.
• The candidate must (a) be over 18 and (b) hold a valid license.

Indicating Plural Possibilities

• The student(s) responsible will be contacted.
• Please submit your document(s) by Friday.

Enclosing Dates, Citations, and References

• Shakespeare (1564–1616) wrote over 37 plays.
• Studies have shown a correlation between sleep and memory (Smith, 2020).

Aside or Editorial Comment

• The meal was delicious (if a bit overpriced).
• He arrived late (as usual) and missed the opening remarks.

Area Codes and Phone Numbers

• Call us at (555) 123-4567.

Square Brackets [ ]: Rules and Uses

Square brackets (called simply "brackets" in American English) have more specialized uses than parentheses. Their most important function is marking editorial changes or additions within quoted material.

Insertions in Quotations

When you need to add clarifying information to a direct quotation, square brackets signal that the addition is yours, not the original author's:

The witness stated, "He [the defendant] was standing near the entrance."

The report noted that "the results [of the clinical trial] exceeded expectations."

Changing Capitalization or Grammar in Quotes

If you need to adjust the grammar of a quotation to fit your sentence, brackets indicate the change:

According to the study, "[t]he primary factor was environmental."

The [sic] Notation

The Latin word sic (meaning "thus" or "so") in brackets indicates that an error in a quotation appeared in the original and is not a transcription mistake:

The email read, "We look forward to recieving [sic] your application."

Brackets Within Parentheses

When you need brackets inside parentheses, use square brackets for the inner enclosure:

(The data [shown in Figure 2] supports this conclusion.)

Ellipses in Brackets

Some academic styles (particularly MLA) enclose the ellipsis in brackets to make clear that the omission is editorial:

"The policy was designed to [. . .] improve overall efficiency."

Phonetic Transcription

In linguistics, square brackets enclose phonetic transcriptions (as opposed to phonemic transcriptions, which use slashes):

• The word "cat" is pronounced [kæt].

Curly Braces { }: Rules and Uses

Curly braces (also called "curly brackets" or simply "braces") are rarely used in standard English prose. Their domain is primarily mathematics, computer science, and certain academic conventions.

Mathematics: Sets

In mathematics, curly braces denote sets—collections of elements:

• The set of primary colors: {red, blue, yellow}
• The set of natural numbers: {1, 2, 3, 4, …}

Programming and Computer Science

In many programming languages (C, C++, Java, JavaScript, Python dictionaries), curly braces define code blocks, objects, or data structures:

function greet() { return "Hello, world!"; }

Music

In musical notation, curly braces (also called "accolades") group staves together, as when the treble and bass clef staves are connected for piano music.

Nested Brackets in Mathematics

In mathematical expressions with multiple levels of nesting, the convention is often: {[( )]}. Curly braces form the outermost layer.

Angle Brackets < >

Angle brackets (also called "chevrons") have specialized uses:

HTML and XML

Angle brackets form the basis of HTML and XML markup: <p>, <div>, <a href="…">.

Email Addresses and URLs

Angle brackets sometimes enclose email addresses or URLs in formal documents to clearly delineate them from surrounding text:

Contact us at <info@example.com>.

Linguistics

In some linguistic traditions, angle brackets enclose graphemes (written characters) as opposed to phonemes or phones:

• The grapheme <c> can represent /k/ or /s/ in English.

Punctuation Placement with Brackets

Knowing where to place other punctuation in relation to brackets is crucial:

Periods

If the parenthetical text is part of a larger sentence, the period goes outside:

The meeting was productive (though it ran overtime).

If the parenthetical text is a complete, standalone sentence, the period goes inside:

(The meeting ran thirty minutes over schedule.)

Commas

A comma never comes before an opening parenthesis. It can come after the closing parenthesis if the sentence grammar requires it:

Incorrect: The manager, (who was new,) called a meeting.

Correct: Although it was unexpected (to say the least), the policy changed.

Question Marks and Exclamation Points

If the question or exclamation is only within the parentheses, place the mark inside:

She finally agreed to the terms (can you believe it?).

If the entire sentence is a question, place the mark outside:

Did you read the memo (the one about parking)?

Nested Brackets

When you need brackets within brackets, the convention varies by context:

In Prose

Use square brackets inside parentheses:

(The data [compiled over three years] shows a clear trend.)

In Mathematics

The standard nesting order from innermost to outermost is: ( ), [ ], { }:

{2 × [3 + (4 − 1)]}

Avoid Excessive Nesting

In writing, if you find yourself nesting brackets more than one level deep, consider restructuring the sentence. Deeply nested brackets are hard to read and usually indicate that the sentence is trying to do too much at once.

British vs. American Terminology

The terminology for brackets differs between British and American English, which can cause confusion:

The key difference: what Americans call "parentheses," the British simply call "brackets." And what Americans call "brackets," the British specify as "square brackets." Be aware of your audience to avoid miscommunication.

Brackets in Academic Writing

Academic writing makes heavy use of both parentheses and square brackets:

• In-text citations: (Smith, 2020) or (Smith, 2020, p. 45)
• Clarifications in quotes: "[The study] confirmed the hypothesis" (Jones, 2019, p. 12).
• Author-date systems: (cf. Brown, 2018; see also Lee, 2021)
• Statistical notation: (M = 4.5, SD = 1.2)

Following the specific bracket conventions of your citation style (APA, MLA, Chicago, Harvard) is essential for academic credibility.

Common Errors

Error 1: Using Square Brackets Instead of Parentheses for Asides

Incorrect: The weather [which had been terrible all week] finally improved.

Correct: The weather (which had been terrible all week) finally improved.

Error 2: Forgetting the Closing Bracket

Always ensure every opening bracket has a matching closing bracket. This is one of the most common proofreading oversights.

Error 3: Overusing Parentheses

Too many parenthetical asides make writing feel cluttered and disjointed. If information is important enough to include, consider integrating it into the main text. If it's not important, consider removing it entirely.

Error 4: Putting a Comma Before a Parenthesis

Incorrect: The director, (Sarah) approved the budget.

Correct: The director (Sarah) approved the budget.

Summary and Key Takeaways

• Parentheses ( ) enclose supplementary information, abbreviations, citations, and asides in general writing.
• Square brackets [ ] mark editorial insertions within quotations, indicate [sic], and serve specialized academic functions.
• Curly braces { } are used in mathematics (sets) and programming (code blocks), rarely in prose.
• Angle brackets < > appear in HTML, URL notation, and linguistics.
• Place periods inside parentheses for standalone sentences, outside when the parenthetical is part of a larger sentence.
• Nest square brackets inside parentheses when multiple levels are needed.
• Be aware of British vs. American terminology differences.

For more on punctuation, explore our guides to the em dash, en dash, and hyphen, comma rules, and the colon.