To show brackets within brackets, use square brackets within parentheses in American writing.
- Cats like cheese (see The Complete Guide to Cats ).
- Apply for a visa at the nearest embassy. (You may also be eligible for a visa on arrival [VOA]).
- Water levels rose for the third consecutive year (accompanied by higher rainfall [Table 1] and temperatures [Table 2]).
In British style, parentheses are used within parentheses (round within round brackets). Avoid using nested parentheses in American writing.
- British: Fauna populations declined in industrial areas. (Pollinators in the study (bees and hummingbirds) were tracked separately.)
- American: Fauna populations declined in industrial areas. (Pollinators in the study [bees and hummingbirds] were tracked separately.)
Brackets within parentheses
Use square brackets to enclose parenthetical information that appears in text already within parentheses. In other words, if you need to show brackets within brackets, use square brackets within parentheses.
- Studies show that pollution affects pollinators (insects and birds [Table 1] that pollinate flowers).
- Boats today come with all sorts of safety features (the most popular of which is the unsinkable floating device [UFD]).
- Indoor plants require light for photosynthesis (see The Green Guide to Houseplants ).
- Use parentheses (or [round] brackets, as they are called in British English) to enclose explanatory information not essential to the grammar or meaning of a sentence.
- What was amazing is that we managed to photograph the elusive dragonbird (see below for photos [the ones I took with my phone are a bit fuzzy]).
Don’t enclose text in brackets if it can be set off easily using commas without loss of clarity. Commas are less disruptive than brackets.
- Poor: Pollution affect pollinators (insects and birds [like bees and hummingbirds] that pollinate flowers).
Better: Pollution affect pollinators (insects and birds, like bees and hummingbirds, that pollinate flowers).
- Poor: Unseasonal showers occurred in September (despite lower overall temperatures [which themselves were unseasonal]).
Better: Unseasonal showers occurred in September (despite lower overall temperatures, which themselves were unseasonal).
Note that square brackets are also used to enclose editorial comments and clarifications. They then appear by themselves (without being enclosed in parentheses).
In British English, round brackets are simply called brackets, with square brackets referred to as such. In American English, round brackets are called parentheses, and square brackets are called brackets.
American vs. British style
In U.S. style, square brackets are used within parentheses, while in British style, round brackets are generally used within round brackets (parentheses in parentheses).
- American: Rainfall increased in September (as forecast by our model [Table 1]).
British: Rainfall increased in September (as forecast by our model (Table 1)).
- American: A pentameter is a line of verse containing five feet. (In poetry, a foot is a fixed combination of stressed [or long] and unstressed [or short] syllables.)
British: A pentameter is a line of verse containing five feet. (In poetry, a foot is a fixed combination of stressed (or long) and unstressed (or short) syllables.)
- American: The Cat Lovers’ Club of Nusquam (CLC [N]) meets on Mondays.
British: The Cat Lovers’ Club of Nusquam (CLC (N)) meets on Mondays.
- American: What you need is a way to reach customers in their sleep by implanting billboards in their dreams (a somnolent advertising strategy [SAS]).
British: What you need is a way to reach customers in their sleep by implanting billboards in their dreams (a somnolent advertising strategy (SAS)).
Don’t use nested parentheses (round brackets within round brackets) in formal American writing. Use square brackets within parentheses instead.
Punctuation around brackets
The same rules apply to punctuating around brackets within parentheses as those to brackets and parentheses in general. Place a punctuation mark (like a period or) inside brackets if it belongs to the bracketed material, and outside otherwise.
- We need to meet urgently. (Please call me if you are free this week [tomorrow?].)
- A quick review shows that this study on arachnids is not as comprehensive as one would expect (scorpions, for example, were not included [why not?]).
- We studied the effects of pollution on local fauna (arachnids were included in the study [spiders, scorpions, etc.]).
Be careful to check where the punctuation mark should go, depending on whether it belongs to the bracketed material, the parenthetical material around it, or to the larger sentence.
- Cats are expert climbers (as discussed by Dash ).
- Cats are expert climbers. (For an excellent analysis of feline climbing abilities, see The Complete Guide to Cats .)
Examples from published content
The following examples show how brackets are used within parentheses in U.S. style.
The natural sciences . . . cite by last name and date of publication (Merton 1957a) and regard first names as a literary indulgence (R. K. Merton, Social Theory and Social Structure [hereafter STSS in text]).— “The End Matter,” New Yorker (October 6, 2003)
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)
(The assortment of pictures here are from Chip Clark [the really good ones] and me and my Iphone [the really blurry ones]).— “Smithsonian Hall of Human Origins: Just Go,” National Geographic (May 15, 2010)
The most predictive neural activity was associated with reward processing (ventral striatum [VS]) and the ability to simulate the minds of others (temporoparietal junction [TPJ] and dorsomedial prefrontal cortex [DMPFC]).— “The Neuroscience of Social Influence,” Scientific American (July 9, 2013)
And these examples illustrate the use of nested parentheses (round brackets within round brackets) in British style.
(Compare neutral (or unmarked) position after the first modal or auxiliary verb: I don’t honestly know.)— Ronald Carter and Michael McCarthy, Cambridge Grammar of English (2008)
At a special delegate conference of the Prison Officers Association Scotland (POA (S)) in Perth on Friday, delegates voted to ballot members for industrial action over pay.— “Scottish Prison Officers’ Union to Ballot for Industrial Action,” BBC News (May 10, 2019)
There is considerable cross-country evidence that banking crises tend to be preceded by unusually strong credit and asset price booms (see below), that those crises go hand-in-hand with permanent output losses (BCBS (2010)), and that subsequent recoveries tend to be slow and protracted (eg Reinhart and Rogoff (2009), Reinhart and Reinhart (2010)).— “Today in Central Banking,” Economist (October 24, 2011)
To enclose text in brackets that appears in material already in parentheses, use square brackets in American writing. In British style, round brackets are used within round brackets (parentheses within parentheses). Remember not to overuse square brackets. If the text can be set off using commas instead, use those instead of square brackets, since they make for smoother reading. The same punctuation rules apply as those for parentheses or brackets in general: punctuation that belongs to the bracketed material goes inside brackets, and outside if it belongs to the text outside brackets.