||This article needs additional citations for verification. (July 2012)|
The vertical bar (|) is a character with various uses in mathematics, where it can be used to represent absolute value, among others; in computing and programming and in general typography, as a divider not unlike the interpunct. It may be called by various other names including the polon, pipe (by the Unix community, referring to the I/O pipeline construct), Sheffer stroke (by computer or mathematical logicians), verti-bar, vbar, stick, vertical line, vertical slash, or bar, think colon, poley, or divider line.
The vertical bar is used as a mathematical symbol in
- absolute value: , read "the absolute value of x".
- norms: , read "the norm of x sub one, x sub two"; though Unicode also provides a special double vertical line symbol U+2016: ‖x‖
- Parallelism in geometry, where indicates that the line is parallel to the line .
- set-builder notation: , read "the set of x such that x is less than two". Often a colon ':' is used instead of a vertical bar.
- cardinality: , read "the cardinality of the set S".
- conditional probability: , read "the probability of X given Y".
- divisibility: , read "a divides b", though Unicode also provides special ‘divides’ and ‘does not divide’ symbols (U+2223 and U+2224: ∣, ∤)
- the Sheffer stroke in logic: , read "a nand b".
- distance: distance notes the shortest distance between dot to line , so is perpendicular to line .
- evaluate (subscript notation): , read "f of x, evaluated at x equals 4" (see subscripts at Wikibooks)
- restriction: denotes a restriction of function where it is defined over a domain which is a superset of .
- Sometimes a vertical bar following a function, with sub- and super-script limits 'a' and 'b' is used when evaluating definite integrals to mean 'f(x) from a to b', or 'f(b)-f(a)'.
- : The quantum mechanical state "".
- : The dual state corresponding to the state above.
- : The inner product of states and .
A pipe is an inter-process communication mechanism originating in Unix which allows the output (standard out and, optionally, standard error) of one process to be used as input (standard in) to another. In this way, a series of commands can be "piped" together, giving users the ability to quickly perform complex multi-stage processing from the command line or as part of a Unix shell script ("bash file"). In most Unix shells (command interpreters), this is represented by the vertical bar character. For example:
where the output from the "egrep" process is piped to the "more" process.
The same "pipe" feature is also found in later versions of DOS and Microsoft Windows.
Specifically, in C and other languages following C syntax conventions, such as C++, Perl, Java and C#,
(a | b) denotes a bitwise or; whilst a double vertical bar
(a || b) denotes a (short-circuited) logical or.
Although not as common as commas or tabs, the vertical bar can be used as a delimiter in a flat file. Examples of a pipe-delimited standard data format are LEDES 1998B and HL7. It is frequently used because vertical bar is typically uncommon in the data itself.
Similarly, the vertical bar may see use as a delimiter for regular expression operations (e.g. in sed). This is useful when the regular expression contains instances of the more common forward slash (
/) delimiter; using a vertical bar eliminates the need to escape all instances of the forward slash.
Backus-Naur form 
In Backus-Naur form, an expression consists of sequences of symbols and/or sequences separated by '|', indicating a choice, the whole being a possible substitution for the symbol on the left.
- <personal-name> ::= <name> | <initial>
Concurrency operator 
In calculi of communicating processes (like pi-calculus), the vertical bar is used to indicate that processes execute in parallel.
Modular arithmetic 
In APL, it is the modulo function (called residue in APL) when between two operands.
Absolute Value 
In APL, it is the absolute value function when before a single operand.
List comprehensions 
Phonetics and orthography 
In the Khoisan languages and the International Phonetic Alphabet, the vertical bar is used to write the dental click (ǀ). A double vertical bar is used to write the alveolar lateral click (ǁ). Since these are technically letters, they have their own Unicode code points in the Latin Extended-B range: U+01C0 for the single bar and U+01C1 for the double bar. Longer single and double vertical bars are used to mark prosodic boundaries in the IPA.
- In the Geneva Bible and early printings of the King James Version, the double vertical bar is used to indicate that an alternative translation is to be found in the margin. Whenever it is used, the marginal note begins with the conjunction "Or".
- In later printings of the King James Version, the double vertical bar may be used to indicate that a comment is to be found in the margin.
The vertical bar ("|") is at position 124 (decimal) in the ASCII character set.
Solid vertical bar vs broken bar 
The broken bar (¦) in computing was historically an allograph of the vertical bar and was perceived so before a broad implementation of extended ASCII character sets (namely, ISO/IEC 8859 series), which did distinguish both. Since 1990s, it is considered a separate character, not a part of ASCII, and also termed "parted rule" in Unicode documentation. But in the text mode fonts, as well as in other TUI applications on DOS, Windows and Unix-like systems, the glyph used for the vertical bar may look exactly like a broken bar. This is no longer the case on Windows 7.
Due to historical confusion between the two, computer keyboards and displays may not clearly or consistently differentiate them. The typical keyboard layout used in the United Kingdom features separate keys for vertical bar and broken bar; however, typically on Windows PCs the vertical bar key produces a broken-bar symbol. Some keyboard drivers map the broken bar key to the vertical bar, and the vertical bar key, shared with the grave accent (`), generates the broken bar when pressed in combination with AltGr.
The broken bar has hardly any practical application and does not appear to have any clearly identified uses distinct from the vertical bar. In non-computing use — for example in mathematics, physics and general typography — the broken bar is not an acceptable substitute for the vertical bar. Aforementioned usages in computing rely on the abstract character with code point 124 (0x7C) in ASCII (or ASCII compatible code page) and do not depend on visual rendering, which actually may be a broken bar in some environments.
In common character maps 
|Vertical bar ('|')||Broken bar ('¦')|
|ASCII||decimal (base-10): 124
hexadecimal (base-16): 7C
|ISO/IEC 8859-1||hexadecimal: 7C||hexadecimal: A6|
|EBCDIC (CCSID 500 variant)||hexadecimal: BB||hexadecimal: A6|
|HTML|||||¦ or ¦|
Additional related Unicode characters:
- Double vertical line ('‖'): U+2016
- Latin letter dental click (⟨ǀ⟩): U+01C0
- Latin letter lateral click (⟨ǁ⟩): U+01C1
- Symbol 'divides' (⟨∣⟩): U+2223
- Various Box-drawing characters at U+2500 to U+257F
In text processing 
In LaTeX, the vertical bar can be used as delimiter in mathematical mode. The sequence \| creates a double vertical line (a | b \| c is set as ). This has different spacing from \mid and \parallel, which are relational operators: a \mid b \parallel c is set as . In LaTeX text mode, the vertical bar produces an em dash (—), or you can use the \textbar command instead.
See also 
- Jim Price (2010-05-24). "ASCII Chart: IBM PC Extended ASCII Display Characters". Retrieved 2012-02-23.
- Jukka "Yucca" Korpela (2006-09-20). "Detailed descriptions of the characters". Retrieved 2012-02-23.
- Broken bar is not considered a part of ASCII since early 1990s