There are four main kinds of brackets: round brackets or parentheses ( ), square brackets [ ], curly brackets or braces { }, and angle brackets ⟨ ⟩. In everyday writing you mostly need the first two: parentheses for extra information and square brackets for changes inside a quotation.
Brackets are used in writing, in mathematics, and in programming, and each field gives them its own jobs. With so many kinds, it is hard to know which one to use where. This article explains each kind and shows the uses that matter for formal, informal, and creative writing.
The different kinds of brackets
In general, brackets are a pair of punctuation marks that enclose a piece of text or a group of numbers and set it apart from the rest of a text or a formula. Slashes and backslashes sometimes work as a kind of bracket too, but the four main kinds below are the ones you meet almost every day, in many different fields.
Round brackets or parentheses ( )
Parentheses (in British English often simply called brackets) are the most common kind of bracket, especially in writing. They appear in formal and informal writing for several purposes:
- To clarify information in a text: Maria (who was late for school) was scolded by the teacher.
- To set information apart from the rest of the text: The term pit bull (usually a mix of bulldog and terrier) often refers to dogs of a certain build rather than to a purebred pit bull.
- To show the singular or plural, or both genders: I don't know if (s)he would be happy to hear that.
Parentheses are also used in other fields:
- Programming: around the arguments of a function, or as the basic building block of a language such as Lisp: (incf x)
- Mathematics: to show which operation comes first, (2 + 5) × 5, or to set the argument of a function apart, f(x)
Sometimes a single closing parenthesis appears on its own, without a partner. This is common in lists that number their items with lowercase letters or numbers: a), b), c).
Square brackets [ ]
In American English, the word "brackets" usually means square brackets. They do different jobs from parentheses:
- To put brackets inside parentheses: The Battle of Hastings (fought between the Norman-French army and the English army [on October 14, 1066]) began the Norman conquest of England.
- To add something to a quotation, or to change it so that it makes sense on its own: "The Dark Lord has Nine. But we have One, mightier than they: the White Rider [Gandalf the White]. He has passed through the fire and the abyss, and they shall fear him." (J.R.R. Tolkien, The Lord of the Rings)
- To mark words left out of a quotation: the ellipsis that stands for the missing words is often put in square brackets, so the reader knows the gap is yours and not the author's.
Square brackets are used in other fields and for other purposes as well:
- Editing and proofreading: to insert a comment or show a change in a sentence, for example [sic], which tells the reader that a mistake was already in the original
- Mathematics: to write a closed interval, [0, 1]
- Chemistry: to show the concentration of a substance, [NaCl]
- Programming: to mark arrays and the position of an item in them, numbers[10]
- Linguistics: to enclose a phonetic transcription, [ˈlɪɾɫ̩] for "little"
Curly brackets or braces { }
Curly brackets are rare in formal and informal writing, but they appear in poetry and in music. In both, they join lines that belong together, such as rhyming lines in a poem or the staves of a piano part.
Curly brackets are far more common in other fields:
- Mathematics: to enclose the members of a set, {5, 10, 15}
- Programming: to enclose a group of statements, in languages such as C, Java, and JavaScript
Angle brackets ⟨ ⟩
Angle brackets are mostly used outside everyday writing:
- Mathematics: to write the inner product of two vectors, ⟨a, b⟩
- Programming: to enclose the tags of HTML and XML, the languages web pages are written in
- Linguistics: to mark how a word is spelled, as opposed to how it sounds, ⟨little⟩
On a keyboard, the less-than sign < and the greater-than sign > often stand in for angle brackets. In French and several other languages, double angle marks (« ») are used as quotation marks.
Even more brackets
Other languages, and some fields of mathematics, physics, and linguistics, use even more kinds of brackets. Most people never need them in their daily writing, so this article does not explain them in detail. You may still have seen some of them:
- Floor and ceiling brackets: ⌊ ⌋ and ⌈ ⌉
- Half brackets: ⸤ ⸥ and ⸢ ⸣
- Double brackets: ⟦ ⟧
- Brackets with quills: ⁅ ⁆