Both parentheses (round brackets) and square brackets enclose additional information. Parentheses set off optional information that may be useful but is tangential to the main meaning of the passage. Square brackets enclose words added to a quote by someone other than the original author. Square brackets are also used in American writing as brackets within parentheses.
When to use parentheses (round brackets)
Use parentheses (round brackets) to enclose any additional or supplementary information that may be useful but is tangential to the main point of the passage.
- Rainfall increased in 2022 (see Table 1).
- Marine mammals (seals, whales, dolphins, etc.) help counteract ocean stratification (the division of water into layers).
- Cats can teleport (which is how they disappear just before a vet visit).
- Cats love pizza. (They like it even better than cheese.)
Information enclosed in parentheses should not be essential either to the meaning or to the grammar of a sentence.
Also use parentheses to provide a definition or enclose abbreviations and their full forms.
- A portable floating device (PFD) can help you travel on water.
- Always wear safety gear on an ATV (all-terrain vehicle).
- In this study, we investigate the habits of monotremes (egg-laying mammals).
Parentheses can also set off an afterthought from the main text.
- A bird in hand is better than two in the bush, they say (depends on the bird, I’d say).
- We should book our flight tickets soon (today, if possible).
- The cat (or perhaps a ghost?) broke the vase.
- It was I who sent the flowers. (I thought you would know.)
Parentheses are also used toin many academic styles.
- Research suggests that breathing while walking can extend your life span (Johnson 2019, 271–2).
- Remiramen Femy, The Truth about Cats (London: Nusquam Publishing, 1997), 97. Dash M. (2009). The green guide to houseplants (Vols. 1–3). New York, NY: Pouthena.
When to use square brackets
Use square brackets to add information to quoted text. Brackets enclose words added to a quote by someone other than the original author (such as editorial comments and clarifications).
- She continued, “It [the photograph] was clearly taken at dawn.”
- “The two presidents [U.S. and French] will meet before the conference in Geneva.”
- Dash explains, “We use the German concept of Bildung [self-cultivation] to help students grow as people rather than turn into career-oriented money-making machines.”
- Cats can control human behavior. [Some behaviorists suggest that cats have the power of mind control; see Dash 1997.—Ed.]
Also enclose the word sic within brackets. Sic indicates that text has been reproduced exactly, including any grammatical or spelling errors.
- The governor said she had “no calms [sic] about sending the refugees back across the border.”
The word sic may be enclosed in either square brackets or parentheses. In formal and academic writing, prefer to use square brackets. In news copy and informal styles, parentheses are often used instead.
- Correct: In 1807 General Ramsey wrote in his war journal: “The perseverence [sic] of the troops is a matter of great personal pride for me.”
Preferred in formal (academic) writing.
- Also correct: In reply he tweeted, “Perseverence (sic) wins in the end.”
Seen in news copy and other less formal styles.
Use square brackets as brackets within brackets: to enclose parenthetical material that appears in text already within parentheses.
- Daytime temperatures broke all historical records in July 2022 (despite higher rainfall [Table 1]).
- Many cats prefer human laps to cat furniture (see The Complete Guide to Cats ).
Like parentheses, brackets are also used in various academic citation styles.
- Christie, Agatha [Mary Westmacott]. Giant’s Bread. Collins, 1930.
- Breathing increases a person’s life span, as experimentally demonstrated by Femy , .
British vs. American style
For brackets within brackets, square brackets are used within parentheses in American writing, while in British writing, parentheses are used within parentheses (round brackets within round brackets).
- American: We tracked weather data across six continents for this study (using standard methodologies [Appendix A]).
British: We tracked weather data across six continents for this study (using standard methodologies (Appendix A)).
- American: We take every measure to keep you safe. (As noted, a UFD [unsinkable floating device] is stored under every seat.)
British: We take every measure to keep you safe. (As noted, a UFD (unsinkable floating device) is stored under every seat.)
In American English, round brackets are called parentheses, while square brackets are usually referred to as just brackets. In British English, round brackets are what are known as brackets, with square brackets known as such.
Examples from published content
Here are some examples from published content that show how parentheses and brackets are used in English.
The introduction to a manuscript does not carry a heading that labels it as the introduction. (The first part of a manuscript is assumed to be the introduction.)— Publication Manual of the American Psychological Association, Sixth Edition (2010)
Shells in the form of wampum (tubular shell beads) were used as money by Native Americans.— “A Brief (and Fascinating) History of Money,” Encyclopedia Britannica (Accessed August 22, 2022)
Having burned bridges with the West and sparked an energy war with Europe, Mr Putin is attempting a pivot east (he’s left himself little choice).— “Vladimir Putin and Xi Jinping: An Increasingly Unequal Relationship,” BBC News (September 16, 2022)
‘I think the biggest thing they [super-deep diamonds] inform us about is the process of subduction,’ says Smith.— Zaria Gorvett, “The ‘Super-Deep’ Royal Diamonds Revealing Earth’s Secrets,” BBC Future (September 23, 2022)
‘Blue . . . signifies vigilence [sic], perseverence [sic] & justice.— “Flag of the United States of America,” Encyclopedia Britannica (Accessed September 8, 2022)
It may be reduced from carbonated aciferous tartrite of oxide of rubidium (in a manner similar to the reduction of kalium [potassium]).— “An Imaginative View of Saturn from Titan in 1915,” Scientific American (March 1, 2015)