Bracket

For other senses of this word, see bracket (disambiguation).

For technical reasons, :) and some similar combinations starting with : redirect here. See emoticon.

Brackets are punctuation marks used in pairs to set apart or interject text within other text. With respect to computer science, the term is sometimes said to only strictly apply to the square or box type.

There are four main types of brackets:

• round brackets or parentheses:  ( )
• square brackets or box brackets:  [ ]
• curly brackets or braces:  { }
• angle brackets or chevrons: 〈 〉

All these forms may be used according to typographical conventions that may vary from publication to publication and may vary even more from language to language. Some typical uses in English texts follow.

History

The angle bracket was the earliest type to appear in English. Desiderius Erasmus coined the term lunula to refer to the rounded parentheses (), recalling the round shape of the moon.

Usage

In addition to referring to the class of all types of brackets, the unqualified word bracket is most commonly used to refer to a specific type of bracket. In modern American usage this is usually the square bracket, whereas in modern British usage it is usually the parenthesis (round bracket).

In American usage, parentheses are usually considered separately from other brackets, and calling them "brackets" at all is unusual, even though they serve a similar function. In more formal usage, "parenthesis" may refer to the entire bracketed text, not just to the punctuation marks used {so all the text in this curly bracket may be said to be a parenthesis}.

Types

Parentheses ( )

Parentheses (singular parenthesis)—sometimes called round brackets, curved brackets, oval brackets, or just brackets; or, colloquially, parens, or fingernails— contain material that could be omitted without destroying or altering the meaning of a sentence.

In formal writing, parentheses may be used to add supplementary information, such as "Sen. Edward Kennedy (D., Massachusetts) spoke at length." They can also indicate shorthand for "either singular or plural" for nouns—e.g., "the claim(s)".

In literature and informal writing, parenthetical phrases have been used extensively in stream of consciousness literature. Of particular note is the southern American author William Faulkner (see Absalom, Absalom! and the Quentin section of The Sound and the Fury). In most writing, overuse of parentheses is usually a sign of a badly structured text. A milder effect may be obtained by using a pair of commas as the delimiter. If the sentence contains commas for other purposes, visual confusion may result.

Historically, parentheses have been used where the slash is currently used—that is, in order to depict alternatives, such as "parenthesis)(parentheses". Examples of this usage can be seen in editions of Fowler’s.

Parentheses may also be nested (with one set (such as this) inside another set). This is not commonly used in formal writing (though sometimes other brackets [especially square brackets] will be used for one or more inner set of parentheses [in other words, secondary {or even tertiary} phrases can be found within the main sentence]).

Any punctuation inside parentheses or other brackets is independent from the rest of the text: "Mrs. Pennyfarthing (What? Yes, that was her name!) was my landlady". In this usage, the explanatory text in the parentheses is a parenthesis. (It is most common for the parenthesized text to be within a single sentence, but not uncommon for an entire sentence, or even several sentences, of supplemental material to be in parenthesis. In this case, even the final full stop would be within the parentheses. Again, the parenthesis implies that the meaning and flow of the text as a whole would be unchanged were the parenthesized sentences removed.)

In mathematics, parentheses signify a different precedence of operators. For example, 2 + 3 × 4 would be 14, since the multiplication is done before the addition. On the other hand, (2 + 3) × 4 is 20, because the parentheses override normal precedence, causing the addition to be done first. They are also used to set apart the arguments in mathematical functions. For example, f(x) is the function f applied to the variable x. In the coordinate system, parentheses denote a set of coordinates. For example, (4,7) may represent the point located at 4 on the x-axis and 7 on the y-axis. Parentheses may also represent intervals. For example, (0,5) is the interval between 0 and 5, not including 0 or 5. Parentheses can also represent multiplication, as in the instance of 2 (3) = 6. Some authors follow the convention in mathematical equations that when parentheses have one level of nesting, the inner pair are parentheses and the outer pair are square brackets. Ex: [5-(7+3)]+4=x. Parentheses may also be used to represent a binomial coefficient.

Box brackets or square brackets [ ]

Square brackets enclose explanatory or missing […] material, especially in quoted text. For example, "I appreciate it [the honor], but I must refuse". Or, "the future of psionics [see definition] is in doubt".

The bracketed expression [sic] is used to indicate errors that are "thus in the original"; a bracketed ellipsis [...] is often used to indicate deleted material; bracketed comments indicate when original text has been modified for clarity: "I'd like to thank [several unimportant people] and my parentals [sic] for their love, tolerance [...] and assistance [italics added]".

In mathematics, square brackets are used in a variety of notations, including standard notations for intervals, commutators, the Lie bracket, and the Iverson bracket.

Square brackets are also sometimes used as parentheses within parentheses (alternating between parentheses and square brackets according to nesting level [though curly braces are often used instead for deeper nesting {as mentioned earlier}]).

With the International Phonetic Alphabet, square brackets indicate a phonetic transcription (as opposed to a phonemic one).

In chemistry, square brackets can also be used to represent the concentration of a chemical substance, or to denote a complex ion.

In architecture, square brackets can be used to emphasize cool words, such as [metaliving] or [cityscape]. It can also be used to make the layout generally look more cool and modern.

In proofreading, square brackets (called move-left symbols or move right symbols) are added to the sides of text to indicate changes in indentation:

Move left	[To Fate I sue, of other means bereft, the only refuge for the wretched left.
Center	]Paradise Lost[
Move up

Curly brackets or braces { }

Curly brackets (so-called in British English; North American English prefers braces) are sometimes used in prose to indicate a series of equal choices: "Select your animal {goat, sheep, cow, horse} and follow me". They are used in specialized ways in poetry and music (to mark repeats or joined lines). The musical terms for this mark joining staves are "accolade" and "brace." In mathematics they delimit sets. In many programming languages, they enclose groups of statements.

Presumably due to the similarity of the words brace and bracket (although they do not share an etymology), many people casually treat brace as a synonym for bracket. Therefore, when it is necessary to avoid any possibility of confusion, such as in computer programming, it may be best to use the term curly bracket rather than brace. However, general usage in North American English favours the latter form. The term curly braces is redundant since no other type of brace exists. Indian programmers often use the name "flower bracket".

Curly brackets are often used in internet communities and through instant messaging to indicate hugging.

Angle brackets or chevrons 〈 〉

Angle brackets (〈 〉; Unicode U+2329 and U+232A) are often used to enclose highlighted material. Some dictionaries use angle brackets to enclose short excerpts illustrating the usage of words. In physical sciences, angle brackets are used to denote an average over time or another continuous parameter. For example,

$\left\langle V(t)^{2}\right\rangle =\lim _{T\to \infty }{\frac {1}{T}}\int _{-T/2}^{T/2}V(t)^{2}{\rm {d}}t.$

In linguistics, angle brackets indicate orthography, as in "The English word /kæt/ is spelled 〈cat〉."

In textual criticism, and hence in many editions of poorly transmitted works, angle brackets denote sections of the text which are illegible or otherwise lost; the editor will often insert his own reconstruction where possible within them.

Angle brackets are infrequently used to denote dialogue that is thought instead of spoken, such as:

〈What a beautiful flower!〉

Single and double angle brackets (〈 〉) or pairs of the appropriate comparison operators (<<, >>) are sometimes used instead of guillemets («, ») (used as quotation marks in many languages) when the proper glyphs are not available.

The mathematical or logical symbols for greater-than (>) and less-than (<) are inequality operators, and are not punctuation marks when so used. Nevertheless, since true angle brackets are not available on a typical computer keyboard, the "less than" and "greater than" symbols are often used instead. These are often loosely referred to as angle brackets when used in this way. For example, the symbols < and > are often used to set apart URLs in text, such as "I found it on Example.com <http://www.example.com/>". It may also often be found to indicate an e-mail address, such as "This photo is copyrighted by John Smith <john@smith.com>", and is the computer-readable form for such in message headers as specified by RFC 2822.

Chevrons are part of standard Chinese, and Korean punctuation, where they generally enclose the titles of books: ︿ and ﹀ or ︽ and ︾ for traditional vertical printing, and 〈 and 〉 or 《 and 》 for horizontal printing.

In comic books, angle brackets are often used to mark dialogue that has been translated notionally from another language.

Angle brackets can also be used to indicate an action or status (eg. <Waves> or <Offline>), particularly in online, real-time text-based discussions (instant messaging, bulletin boards, etc). (Here, asterisks can also be used to signify an action.)

In computing

Various brackets in Arial

• Opening and closing parentheses correspond to ASCII and Unicode characters 40 and 41, or U+0028 and U+0029, respectively.
• For square brackets corresponding values are 91 and 93, or U+005B and U+005D.
• For braces, 123 and 125, or U+007B and U+007D.
• True angle brackets are available in Unicode at code points U+27e8 and U+27e9 (for mathematical use), or U+9001 and U+9002 (for East Asian languages), or U+2329 and U+232A (for "technical" use, canonically equivalent to the CJL code points U+900…). The less than and greater than symbols can be found in both Unicode and ASCII at code points 60 and 62, or U+003C and U+003E.

Also, in many computer languages:

• "(" and ")"
  • are often used to define the syntactic structure of expressions, overriding operator precedence: a*(b+c) has subexpressions a and b+c, whereas a*b+c has subexpressions a*b and c.
  • contain the arguments to functions: substring($val,10,1). On the other hand, in Lisp they open and close s-expressions and therefore function applications: (cons a b). In Fortran-family languages, they are also used for array references.
  • in the Perl programming language, they are used to define lists, static array-like structures; this idiom is extended to their use as containers of subroutine (function) arguments
• "[" and "]"
  • refer to array or associative array elements, and sometimes to define the number of elements in an array: queue[3].
  • in Perl, they create anonymous arrays and refer to array elements as above.
  • in most regular expression syntaxes square brackets denote a character class: a series of possible characters to choose from.
  • are often used in functional languages (such as Standard ML) for list literals.
• "{" and "}"
  • are used in some languages to define the beginning and ending of blocks of code. Languages which use this convention are said to belong to the curly brace family of programming languages.
  • denote array constants.
  • in the Pascal programming language, they define the beginning and ending of comments.
  • in most regular expression syntaxes, they are used as quantifiers, matching n repetitions of the previous group.
  • in Perl, they create anonymous associative arrays and refer to associative array elements.
• "<" and ">"
  • in SGML (and its applications and variants such as HTML and XML), used to enclose code tags: <div>
  • in Java, C++ and C#, used to delimit generic arguments and preprocessor directives.
  • in Perl, used to read a line from an input source.
  • also used as operators for redirection in Unix' and other operating systems' shells. In this context, they are often referred to as hoinkies (singular hoinky) in order to "avoid confusion with other bracket-type operators". Template:Wikiref

Layout styles

In normal text an opening bracket is not put at the end of a line, and a closing bracket not at the beginning. However, in computer code this is often done to aid readability. For example, a bracketed list of items separated by semicolons may be written with the brackets on separate lines, and the items, followed by the semicolon, each on one line.

For example, the CSS code

h1 { font-weight: bold; font-size: 12pt; line-height: 14pt }

may also be written

h1 {
  font-weight: bold;
  font-size: 12pt;
  line-height: 14pt
}

See Indent style.

In mathematics

Main article: Bracket (mathematics)

In addition to the use of parentheses to specify the order of operations, both parentheses and square brackets are used to denote an interval. The notation [a, c) is used to indicate an interval from a to c that is inclusive of a but exclusive of c. That is, [5, 12) would be the set of all real numbers between 5 and 12, including 5 but not 12. The numbers may come as close as they like to 12, including 11.999 and so forth (with any finite number of 9s), but 12.0 is not included. (In Europe, the notation [5,12[ is also used for this.) The endpoint adjoining the square bracket is known as closed, while the endpoint adjoining the parenthesis is known as open. If both types of brackets are the same, the entire interval may be referred to as closed or open as appropriate. Whenever infinity or negative infinity is used as an endpoint, it is always considered open and adjoined to a parenthesis. See Interval (mathematics) for a more complete treatment.

In quantum mechanics, angle brackets are also used as part of Dirac's formalism, bra-ket notation, to note vectors from the dual spaces of the Bra <A| and the Ket |B>. Mathematicians will also commonly write <a,b> for the inner product of two vectors. In statistical mechanics, angle brackets denote ensemble or time average. Angle brackets are used in group theory to write group presentations, and to denote the subgroup generated by a collection of elements.

In group theory and ring theory, square brackets denote the commutator. In group theory, the commutator [g,h] is commonly defined as g⁻¹h⁻¹gh. In ring theory, the commutator [a,b] is defined as ab − ba. Furthermore, in ring theory, braces denote the anticommutator where {a,b} is defined as ab + ba. The square bracket is also used to denote the Lie derivative, or more generally the Lie bracket in any Lie algebra.

Various notations, like the vinculum have a similar effect to brackets in specifying order of operations, or otherwise grouping several characters together for a common purpose.

In the Z formal specification language, curly braces define a set and angle brackets define a sequence.

In accounting

Traditionally in accounting, negative amounts are placed in parentheses.

In law

Brackets are used in the citation of law reports to identify parallel citations to non-official reporters. For example: Chronicle Pub. Co. v. Superior Court, (1998) 54 Cal.2d 548, [7 Cal.Rptr. 109]. Likewise, when quoted material is in any way altered, the alterations are enclosed in brackets within the quotation. For example: Plaintiff asserts his cause is just, stating, "[m]y causes is [sic] just." While in the original quoted sentence the word "my" was capitalized, it has been modified in the quotation and the change signalled with brackets. Similarly, where the quotation contained a grammatical error, the quoting author signalled that the error was in the original with "[sic]" (Latin for "thus"). California Style Manual, section 4:59 (4th ed.)

In sports

Tournament brackets, the diagrammatic representation of the series of games played during a tournament usually leading to a single winner, are so named for their resemblance to square brackets.

See also

• Emoticon
• Japanese typographic symbols

Bibliography

• Bryant, Randal E.; David, O'Hallaron (2003), Computer Systems: A Programmer's Perspective (2003 ed.), Upper Saddle River, NJ: Pearson Education, p. 794, ISBN 0-13-034074-X
• Lennard, John (1991). But I Digress: The Exploitation of Parentheses in English Printed Verse. ISBN 0-19-811247-5.
• Turnbull (1964). The Graphics of Communication. New York: Holt. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) States that what are depicted as square brackets above are called braces and curly brackets are called brackets. This was the terminology in US printing prior to computers.