Bracket
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| Punctuation marks |
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apostrophe ( ' ) ( ’ ) |
| Other typographer's marks |
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ampersand ( & ) |
- See parenthesis for an account of the rhetorical concept from which the name of the punctuation mark is derived.
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In writing
Brackets are punctuation marks, used in pairs to set apart or interject text within other text. Types of brackets include parentheses ( ), (the singular is parenthesis) square brackets [ ], braces { }, and angle brackets 〈 〉. 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.
Types of brackets
Parentheses ( )
Parentheses are used to contain parenthetical (or optional, additional) material in a sentence that could be removed without destroying the meaning of the main text. For example, "George Washington (the father of his country) was not the wooden figure with wooden teeth that many think him". Indeed, such an interjection is called a parenthesis, and may also be set off with dashes or commas. Overuse of parentheses is usually a sign of a badly structured text.
Parentheses may be used to add supplementary information, such as "Sen. Kennedy (D., Massachusetts) spoke at length".
Historically, parentheses have been used in place of the slash in order to depict alternatives, such as "parenthesis)(parentheses".
Parentheses may also be nested (with one set inside another set (this is not commonly used in formal writing)). Sometimes square brackets will be used for the inner set of parentheses (in other words, a secondary phrase in parentheses).
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 mathematics, parentheses are used to signify a different precedence of operators. For example, 2 + 3 × 4 would be 14, since the multiplication is done before the addition. (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 to mathematical functions. For example, f(x) is the function f applied to the variable x. In the coordinate system, parentheses are used to denote a set of coordinates. For example, (4,7) represents the point located at 4 on the x-axis and 7 on the y-axis.
Parentheses are sometimes called round brackets, curved brackets or, colloquially, parens, or fingernails. John Lennard (in "The exploitation of parentheses in English printed verse") usefully coined the term lunula to refer specifically to the opening curved bracket, the closing curved bracket and the textual contents between.
Square brackets [ ]
Square brackets are used to 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] (Latin for "thus") is used to indicate errors that are "thus in the original"; a bracketed ellipsis [...] is used to indicate deleted material; bracketed comments are used to 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]".
Square brackets are also sometimes used as parentheses within parentheses - "secondary parentheses" as mentioned earlier.
With the International Phonetic Alphabet, square brackets are used to 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 wikis like Wikipedia, double square brackets ( [[ ]] ) are used to form wiki links to other pages.
Curly brackets or braces { }
Curly brackets (so-called in European 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). In mathematics they are used to delimit sets.
Presumably due to the similarity of the words brace and bracket (they 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 favors the latter form. The term curly braces is redundant since that is the only kind of braces there are.
In computer programming, curly brackets sometimes denote the beginning and ending of a sequence of statements.
Angle brackets or Chevrons 〈 〉
Angle brackets (〈, 〉) are often used to enclose highlighted material. Some dictionaries use angle brackets to enclose short excerpts illustrating the usage of words. True angle brackets are not available on a typical computer keyboard, so the "less than" and "greater than" symbols are 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 in Wikipedia <http://www.wikipedia.org/>".
Angle brackets are used in mathematics and logic to delimit ordered n-tuples.
Single and double angle brackets (〈〈, 〉〉) or pairs of the appropriate comparison operators (<<, >>) are sometimes used instead of guillemets when the proper glyphs are not available.
The mathematical or logical symbols for greater-than (>) and less-than (<), when used as such, are not punctuation marks.
In computing
- Opening and closing parentheses correspond to Unicode and ASCII characters 40 and 41, or 0x0028 and 0x0029, respectively.
- For square brackets corresponding values are 91 and 93, or 0x005B and 0x005D.
- For braces, 123 and 125, or 0x007B and 0x007D.
- True angle brackets are available in Unicode at code points 9001 and 9002, or 0x2329 and 0x232A. The less than and greater than symbols can be found in both Unicode and ASCII at code points 60 and 62 respectively, or 0x003C and 0x003E.
Also, in many computer languages:
- "(" and ")" are used to contain the arguments to functions:
substring($val,10,1). Parentheses are so ubiquitous in the Lisp programming language that the name is said to be an acronym for "Lots of Irritating Superfluous Parentheses". They may also be used to indicate the start and end of lists. - "[" and "]" are used to define the number of elements in an array, or reference one of those elements:
$queue[3]. In MediaWiki's syntax, a double square-bracket set is used to make a wikilink to the term defined inside:[[Bracket]]. - "{" and "}" are used to define the beginning and ending of blocks of code or define the initial contents of an array. To complicate things, in the Pascal programming language, "{" and "}" define the beginning and ending of comments. Languages which use the former convention are said to belong to the Curly brace family.
- "<" and ">" are used in SGML (and other formats based on SGML, such as HTML and XML), to enclose code tags:
<div>.
Layout rules
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 semi-colons may be written with the brackets on separate lines, and the items, followed by the semicolon, each on one line.
For example
H1 { font-weight: bold; font-size: 12pt; line-height: 14pt }
may also be written
H1 {
font-weight: bold;
font-size: 12pt;
line-height: 14pt
}
A superfluous semicolon may be added after the last item for uniformity of the item lines.
See: Indent style
In mathematics
In addition to the use of parentheses to specify the order of operations, both parentheses and square brackets can also be used to denote an interval. The notation [a, c) is used to indicate a sequence 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 except 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.
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.
This is used in mathematical notation, and appears in some computer programming languages. See the article Interval (mathematics) for a more complete treatment of the subject.
In quantum mechanics, brackets are also used as part of Dirac's formalism to note vectors form the dual spaces of the Bra <A| and the Ket |B>.
In sports
- Tournament brackets, such as those used in sports tournaments (tennis tournaments, 9-ball tournaments, or March Madness and other NCAA tournaments).
In mechanics and structures
A bracket may be an inverted "L" shape, such as is usually used to hold up a shelf, or a rafter extension and its diagonal brace supporting an overhanging roof over a gable. Decorative brackets used in furniture and mantlepieces are called corbels.
Reference
Turnbull et al., The Graphics of Communication, Holt, New York: 1964 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.