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The symbol is known variously in English-speaking regions as the number sign, hash, or pound sign. The symbol has historically been used for a wide range of purposes including the designation of an ordinal number and as a ligatured abbreviation for pounds avoirdupois – having been derived from the now-rare . Since 2007, widespread usage of the symbol to introduce
metadata tag In information systems, a tag is a keyword or term assigned to a piece of information (such as an Internet bookmark, multimedia, database record, or computer file). This kind of metadata helps describe an item and allows it to be found again ...
s on
social media Social media are interactive media technologies that facilitate the creation and sharing of information, ideas, interests, and other forms of expression through virtual communities and networks. While challenges to the definition of ''social medi ...
platforms has led to such tags being known as "
hashtag A hashtag is a metadata tag that is prefaced by the hash (also known as pound or octothorpe) sign, ''#''. On social media, hashtags are used on microblogging and photo-sharing services such as Twitter or Instagram as a form of user-generated ...
s", and from that, the symbol itself is sometimes called a hashtag. The symbol is distinguished from similar symbols by its combination of level horizontal strokes and right-tilting vertical strokes.


History

It is believed that the symbol traces its origins to the symbol , an abbreviation of the Roman term '' libra pondo'', which translates as "pound weight". This abbreviation was printed with a dedicated
ligature Ligature may refer to: * Ligature (medicine), a piece of suture used to shut off a blood vessel or other anatomical structure ** Ligature (orthodontic), used in dentistry * Ligature (music), an element of musical notation used especially in the me ...
type element, with a horizontal line across, so that the lowercase letter would not be mistaken for the numeral . Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two slash-like strokes "//". Examples of it being used to indicate pounds exist at least as far back as 1850. The symbol is described as the "number" character in an 1853 treatise on
bookkeeping Bookkeeping is the recording of financial transactions, and is part of the process of accounting in business and other organizations. It involves preparing source documents for all transactions, operations, and other events of a business. Tr ...
, and its double meaning is described in a bookkeeping text from 1880. The instruction manual of the Blickensderfer model 5 typewriter () appears to refer to the symbol as the "number mark". Some early-20th-century U.S. sources refer to it as the "number sign", although this could also refer to the
numero sign The numero sign or numero symbol, №, (also represented as Nº, No, No. or no.), is a Typography, typographic abbreviation of the word ''number''(''s'') indicating ordinal number, ordinal numeration, especially in names and titles. For exampl ...
. A 1917 manual distinguishes between two uses of the sign: "number (written before a figure)" and "pounds (written after a figure)". The use of the phrase "pound sign" to refer to this symbol is found from 1932 in U.S. usage. The term ''hash sign'' is found in South African writings from the late 1960s and from other non-North-American sources in the 1970s. The symbol appears to have been used primarily in handwritten material; in the printing business, the numero symbol (№) and barred-lb () are used for "number" and "pounds" respectively. For mechanical devices, the symbol appeared on the keyboard of the
Remington Remington may refer to: Organizations * Remington Arms, American firearms manufacturer * Remington Rand, American computer manufacturer * Remington Products, American manufacturer of shavers and haircare products * Remington College, American c ...
Standard typewriter (c. 1886) but was not used on the keyboards used for typesetting. It appeared in many of the early teleprinter codes and from there was copied to
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
, which made it available on computers and thus caused many more uses to be found for the character. The symbol was introduced on the bottom right button of
touch-tone Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed ...
keypads in 1968, but that button was not extensively used until the advent of large scale
voicemail A voicemail system (also known as voice message or voice bank) is a computer-based system that allows users and subscribers to exchange personal voice messages; to select and deliver voice information; and to process transactions relating to ind ...
(PBX systems, etc.) in the early 1980s. One of the uses in computers was to label the following text as having a different interpretation (such as a command or a comment) from the rest of the text. It was adopted for use within internet relay chat (
IRC Internet Relay Chat (IRC) is a text-based chat system for instant messaging. IRC is designed for group communication in discussion forums, called '' channels'', but also allows one-on-one communication via private messages as well as chat an ...
) networks circa 1988 to label groups and topics. This usage inspired
Chris Messina Christopher Messina (born August 11, 1974)Hochberg, Mina.Chris Messina stars in 'Ruby Sparks'. ''Newsday'' (Long Island, New York). August 8, 2012: “...the actor, who turns 38 Saturday ugust 11, 2012 After graduating from Northport ew Yor ...
to propose a similar system to be used on
Twitter Twitter is an online social media and social networking service owned and operated by American company Twitter, Inc., on which users post and interact with 280-character-long messages known as "tweets". Registered users can post, like, and ...
to tag topics of interest on the microblogging network; this became known as a
hashtag A hashtag is a metadata tag that is prefaced by the hash (also known as pound or octothorpe) sign, ''#''. On social media, hashtags are used on microblogging and photo-sharing services such as Twitter or Instagram as a form of user-generated ...
. Although used initially and most popularly on Twitter, hashtag use has extended to other social media sites.


Names

;Number sign :'Number sign' is the name chosen by the
Unicode consortium The Unicode Consortium (legally Unicode, Inc.) is a 501(c)(3) non-profit organization incorporated and based in Mountain View, California. Its primary purpose is to maintain and publish the Unicode Standard which was developed with the intenti ...
. Most common in Canada and the northeastern United States. American telephone equipment companies which serve Canadian callers often have an option in their programming to denote
Canadian English Canadian English (CanE, CE, en-CA) encompasses the varieties of English native to Canada. According to the 2016 census, English was the first language of 19.4 million Canadians or 58.1% of the total population; the remainder spoke French ( ...
, which in turn instructs the system to say ''number sign'' to callers instead of ''pound''. ;Pound sign or pound :'Pound sign' or 'pound' are the most common names used in the United States, where the '#' key on a phone is commonly referred to as the ''pound key'' or simply ''pound''. Dialing instructions to an extension such as #77, for example, can be read as "pound seven seven". This name is rarely used outside the United States, where the term ''pound sign'' is understood to mean the currency symbol £. ;Hash, hash mark, hashmark :In the United Kingdom, and some other countries, it is generally called a 'hash' (probably from 'hatch', referring to cross-
hatching Hatching (french: hachure) is an artistic technique used to create tonal or shading effects by drawing (or painting or scribing) closely spaced parallel lines. (It is also used in monochromatic representations of heraldry to indicate what the ...
, although the exact derivation is disputed). :Programmers also use this term; for instance is "hash, bang" or "shebang". ;
Hashtag A hashtag is a metadata tag that is prefaced by the hash (also known as pound or octothorpe) sign, ''#''. On social media, hashtags are used on microblogging and photo-sharing services such as Twitter or Instagram as a form of user-generated ...
:Derived from the previous, the word 'hashtag' is often used when reading social media messages aloud, indicating the start of a hashtag. For instance, the text "#foo" is often read out loud as "hashtag foo" (as opposed to "hash foo"). This leads to the common belief that the symbol itself is called ''hashtag''. Twitter documentation refers to it as "the hashtag symbol". ;Hex :'Hex' is commonly used in Singapore and Malaysia, as spoken by many recorded telephone directory-assistance menus: "Please enter your phone number followed by the 'hex' key". The term 'hex' is discouraged in Singapore in favour of 'hash'. In Singapore, a hash is also called 'hex' in apartment addresses, where it precedes the floor number. ;, octothorpe, octathorp, octatherp : Most scholars believe the word was invented by workers at the
Bell Telephone Laboratories Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mul ...
by 1968, who needed a word for the symbol on the
telephone keypad A telephone keypad is a keypad installed on a push-button telephone or similar telecommunication device for dialing a telephone number. It was standardized when the dual-tone multi-frequency signaling (DTMF) system was developed in the Bell Syste ...
. Don MacPherson is said to have created the word by combining ''octo'' and the last name of
Jim Thorpe James Francis Thorpe ( Sac and Fox (Sauk): ''Wa-Tho-Huk'', translated as "Bright Path"; May 22 or 28, 1887March 28, 1953) was an American athlete and Olympic gold medalist. A member of the Sac and Fox Nation, Thorpe was the first Native ...
, an Olympic medalist. Howard Eby and Lauren Asplund claim to have invented the word as a joke in 1964, combining ''octo'' with the syllable ''therp'' which, because of the "th" digraph, was hard to pronounce in different languages. ''The Merriam-Webster New Book of Word Histories'', 1991, has a long article that is consistent with Doug Kerr's essay, which says "octotherp" was the original spelling, and that the word arose in the 1960s among telephone engineers as a joke. Other hypotheses for the origin of the word include the last name of
James Oglethorpe James Edward Oglethorpe (22 December 1696 – 30 June 1785) was a British soldier, Member of Parliament, and philanthropist, as well as the founder of the colony of Georgia in what was then British America. As a social reformer, he hoped to re ...
or using the Old English word for village, ''
thorp ''Thorp'' is a Middle English word for a hamlet or small village. Etymology The name can either come from Old Norse ''þorp'' (also ''thorp''), or from Old English (Anglo-Saxon) ''þrop''. There are many place names in England with the suffi ...
'', because the symbol looks like a village surrounded by eight fields. The word was popularized within and outside Bell Labs. The first appearance of "octothorp" in a US patent is in a 1973 filing. This patent also refers to the six-pointed asterisk (✻) used on telephone buttons as a "sextile". ;
Sharp Sharp or SHARP may refer to: Acronyms * SHARP (helmet ratings) (Safety Helmet Assessment and Rating Programme), a British motorcycle helmet safety rating scheme * Self Help Addiction Recovery Program, a charitable organisation founded in 19 ...
: Use of the name 'sharp' is due to the symbol's resemblance to , the glyph used in
music notation Music is generally defined as the art of arranging sound to create some combination of form, harmony, melody, rhythm or otherwise expressive content. Exact definitions of music vary considerably around the world, though it is an aspect ...
(). The same derivation is seen in the name of the
Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
programming languages C#, J# and F#. Microsoft says, "It's not the 'hash' (or pound) symbol as most people believe. It's actually supposed to be the musical sharp symbol. However, because the sharp symbol is not present on the standard keyboard, it's easier to type the hash symbol (#). The name of the language is, of course, pronounced 'see sharp'." According to the ECMA-334 C# Language Specification, section 6, ''Acronyms and abbreviations,'' the name of the language is written "C#" ("LATIN CAPITAL LETTER C (U+0043) followed by the NUMBER SIGN # (U+0023)") and pronounced "C Sharp". ;Square :On telephones, the
International Telecommunication Union The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
specification ITU-T E.161 3.2.2 states: "The symbol may be referred to as the square or the most commonly used equivalent term in other languages." Formally, this is not a number sign but rather another character, the
Viewdata square Viewdata is a Videotex implementation. It is a type of information retrieval service in which a subscriber can access a remote database via a common carrier channel, request data and receive requested data on a video display over a separate ...
. The real or virtual keypads on almost all modern telephones use the simple instead, as does their documentation. ;Other :Names that may be seen include:
crosshatch Hatching (french: hachure) is an artistic technique used to create tonal or shading effects by drawing (or painting or scribing) closely spaced parallel lines. (It is also used in monochromatic representations of heraldry to indicate what the t ...
, crunch, fence, flash, garden fence, garden gate, gate, grid, hak, mesh, oof, pig-pen, punch mark, rake, scratch, scratch mark,
tic-tac-toe Tic-tac-toe (American English), noughts and crosses (Commonwealth English), or Xs and Os (Canadian or Irish English) is a paper-and-pencil game for two players who take turns marking the spaces in a three-by-three grid with ''X'' or ''O''. T ...
, and unequal.


Usage

When # prefixes a number, it is read as "number". A "#2 pencil", for example, indicates "a number-two pencil". The abbreviations 'No.' and '№' are used commonly and interchangeably. When # is after a number, it is read as "pound" or "pounds", meaning the unit of weight. The text "5# bag of flour" would mean "five pound bag of flour". The abbreviations "lb." and "" are used commonly and interchangeably. But it is ''not'' a replacement for '£'. The latter usage is rare outside North America. The sign is not used to denote pounds as weight ( or is used for this), and certainly not for pounds currency. The use of as an abbreviation for "number" is common in informal writing, but use in print is rare. Where Americans might write "Symphony #5", British and Irish people usually write "Symphony No. 5". British typewriters and keyboards have a key where American keyboards have a key. Many computer and teleprinter codes (such as BS 4730 (the UK national variant of the
ISO/IEC 646 ISO/IEC 646 is a set of International Organization for Standardization, ISO/International Electrotechnical Commission, IEC standards, described as ''Information technology — ISO 7-bit coded character set for information interchange' ...
character set) substituted '£' for '#' to make the British versions, thus it was common for the same binary code to display as on US equipment and on British equipment. ('$' was not substituted due to obvious problems if an attempt was made to communicate monetary values.)


Mathematics

* In
set theory Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, is mostly conce ...
, #''S'' is one possible notation for the
cardinality In mathematics, the cardinality of a set is a measure of the number of elements of the set. For example, the set A = \ contains 3 elements, and therefore A has a cardinality of 3. Beginning in the late 19th century, this concept was generalized ...
or size of the
set Set, The Set, SET or SETS may refer to: Science, technology, and mathematics Mathematics *Set (mathematics), a collection of elements *Category of sets, the category whose objects and morphisms are sets and total functions, respectively Electro ...
''S'', instead of , S, . That is, for a set S = \, in which all s_i are mutually distinct, \#S = n = , S, . This notation is only sometimes used for
finite set In mathematics, particularly set theory, a finite set is a set that has a finite number of elements. Informally, a finite set is a set which one could in principle count and finish counting. For example, :\ is a finite set with five elements. Th ...
s, usually in
number theory Number theory (or arithmetic or higher arithmetic in older usage) is a branch of pure mathematics devoted primarily to the study of the integers and arithmetic function, integer-valued functions. German mathematician Carl Friedrich Gauss (1777 ...
, to avoid confusion with the
divisibility In mathematics, a divisor of an integer n, also called a factor of n, is an integer m that may be multiplied by some integer to produce n. In this case, one also says that n is a multiple of m. An integer n is divisible or evenly divisible by ...
symbol, e.g. a \mid b. * In
topology In mathematics, topology (from the Greek language, Greek words , and ) is concerned with the properties of a mathematical object, geometric object that are preserved under Continuous function, continuous Deformation theory, deformations, such ...
, ''A''#''B'' is the
connected sum In mathematics, specifically in topology, the operation of connected sum is a geometric modification on manifolds. Its effect is to join two given manifolds together near a chosen point on each. This construction plays a key role in the classifi ...
of
manifold In mathematics, a manifold is a topological space that locally resembles Euclidean space near each point. More precisely, an n-dimensional manifold, or ''n-manifold'' for short, is a topological space with the property that each point has a n ...
s ''A'' and ''B'', or of knots ''A'' and ''B'' in
knot theory In the mathematical field of topology, knot theory is the study of knot (mathematics), mathematical knots. While inspired by knots which appear in daily life, such as those in shoelaces and rope, a mathematical knot differs in that the ends are ...
. * In
number theory Number theory (or arithmetic or higher arithmetic in older usage) is a branch of pure mathematics devoted primarily to the study of the integers and arithmetic function, integer-valued functions. German mathematician Carl Friedrich Gauss (1777 ...
, ''n''# is the
primorial In mathematics, and more particularly in number theory, primorial, denoted by "#", is a function from natural numbers to natural numbers similar to the factorial function, but rather than successively multiplying positive integers, the function ...
of ''n''. * In
constructive mathematics In the philosophy of mathematics, constructivism asserts that it is necessary to find (or "construct") a specific example of a mathematical object in order to prove that an example exists. Contrastingly, in classical mathematics, one can prove th ...
, # denotes an
apartness relation In constructive mathematics, an apartness relation is a constructive form of inequality, and is often taken to be more basic than equality. It is often written as \# (⧣ in unicode) to distinguish from the negation of equality (the ''denial inequ ...
. * In
computational complexity theory In theoretical computer science and mathematics, computational complexity theory focuses on classifying computational problems according to their resource usage, and relating these classes to each other. A computational problem is a task solved by ...
, #P denotes a
complexity class In computational complexity theory, a complexity class is a set of computational problems of related resource-based complexity. The two most commonly analyzed resources are time and memory. In general, a complexity class is defined in terms of ...
of counting problems. The standard notation for this class uses the number sign symbol, not the sharp sign from music, but it is pronounced "sharp P". More generally, the number sign may be used to denote the class of counting problems associated with any class of search problems.


Computing

* In
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expre ...
and
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
, the symbol has a
code point In character encoding terminology, a code point, codepoint or code position is a numerical value that maps to a specific character. Code points usually represent a single grapheme—usually a letter, digit, punctuation mark, or whitespace—but ...
as and in
HTML5 HTML5 is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and final major HTML version that is a World Wide Web Consortium (W3C) recommendation. The current specification is known as the HTML ...
. * In many scripting languages and data file formats, especially ones that originated on Unix, introduces a comment that goes to the end of the line. The combination at the start of an executable file is a " shebang", "hash-bang" or "pound-bang", used to tell the operating system which program to use to run the script (see magic number). This combination was chosen so it would be a comment in the scripting languages. ** is the symbol of the CrunchBang Linux distribution. * In the
Perl Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...
programming language, is used as a modifier to array syntax to return the index number of the last element in the
array An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
, e.g., an array's last element is at $array #array/nowiki>. The number of elements in the array is , since Perl arrays default to using zero-based indices. If the array has not been defined, the return is also undefined. If the array is defined but has not had any elements assigned to it, e.g., , then returns . See the section on Array functions in the Perl language structure article. * In both the C and
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
preprocessors, as well as in other syntactically C-like languages, is used to start a preprocessor directive. Inside macros, after , it is used for various purposes; for example, the double pound (hash) sign is used for token
concatenation In formal language, formal language theory and computer programming, string concatenation is the operation of joining character string (computer science), character strings wikt:end-to-end, end-to-end. For example, the concatenation of "sno ...
. * In
Unix shell A Unix shell is a command-line Interpreter (computing), interpreter or shell (computing), shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting langua ...
s, is placed by convention at the end of a
command prompt Command Prompt, also known as cmd.exe or cmd, is the default command-line interpreter for the OS/2, eComStation, ArcaOS, Microsoft Windows (Windows NT family and Windows CE family), and ReactOS operating systems. On Windows CE .NET 4.2, W ...
to denote that the user is working as
root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
. * is used in a URL of a web page or other resource to introduce a "
fragment identifier In computer hypertext, a URI fragment is a character string (computer science), string of character (computing), characters that refers to a resource (computer science), resource that is subordinate to another, primary resource. The primary resou ...
" – an id which defines a position within that resource. In HTML, this is known as an anchor link. For example, in the URL the portion after the () is the fragment identifier, in this case denoting that the display should be moved to show the tag marked by in the HTML. *
Internet Relay Chat Internet Relay Chat (IRC) is a text-based chat system for instant messaging. IRC is designed for group communication in discussion forums, called ''channels'', but also allows one-on-one communication via private messages as well as chat and ...
: on (IRC) servers, precedes the name of every
channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austral ...
that is available across an entire IRC network. * In
blog A blog (a truncation of "weblog") is a discussion or informational website published on the World Wide Web consisting of discrete, often informal diary-style text entries (posts). Posts are typically displayed in reverse chronological order ...
s, is sometimes used to denote a
permalink A permalink or permanent link is a URL that is intended to remain unchanged for many years into the future, yielding a hyperlink that is less susceptible to link rot. Permalinks are often rendered simply, that is, as clean URLs, to be easier to ...
for that particular weblog entry. * In
lightweight markup language A lightweight markup language (LML), also termed a simple or humane markup language, is a markup language with simple, unobtrusive syntax. It is designed to be easy to write using any generic text editor and easy to read in its raw form. Lightwei ...
s, such as
wiki A wiki ( ) is an online hypertext publication collaboratively edited and managed by its own audience, using a web browser. A typical wiki contains multiple pages for the subjects or scope of the project, and could be either open to the pu ...
text, is often used to introduce numbered list items. * is used in the
Modula-2 Modula-2 is a structured, procedural programming language developed between 1977 and 1985/8 by Niklaus Wirth at ETH Zurich. It was created as the language for the operating system and application software of the Lilith personal workstation. It w ...
and
Oberon Oberon () is a king of the fairies in medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania, Queen of the Fair ...
programming languages designed by
Niklaus Wirth Niklaus Emil Wirth (born 15 February 1934) is a Swiss computer scientist. He has designed several programming languages, including Pascal (programming language), Pascal, and pioneered several classic topics in software engineering. In 1984, he w ...
and in the Component Pascal language derived from Oberon to denote the ''not equal'' symbol, as a stand-in for the mathematical unequal sign , being more intuitive than or . For example: * In
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH ...
, is used for attributes such as in . * In
OCaml OCaml ( , formerly Objective Caml) is a general-purpose programming language, general-purpose, multi-paradigm programming language which extends the Caml dialect of ML (programming language), ML with object-oriented programming, object-oriented ...
, is the operator used to call a method. * In
Common Lisp Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fro ...
, is a dispatching
read macro In the programming language Lisp, the reader or read function is the parser which converts the textual form of Lisp objects to the corresponding internal object structure. In the original Lisp, S-expressions consisted only of symbols, integers, ...
character used to extend the
S-expression In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested list (tree-structured) data. S-expressions were invented for and popularized by the programming la ...
syntax with short cuts and support for various data types (complex numbers, vectors and more). * In
Scheme A scheme is a systematic plan for the implementation of a certain idea. Scheme or schemer may refer to: Arts and entertainment * ''The Scheme'' (TV series), a BBC Scotland documentary series * The Scheme (band), an English pop band * ''The Schem ...
, is the prefix for certain syntax with special meaning. * In
Standard ML Standard ML (SML) is a general-purpose, modular, functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of the ...
, , when prefixed to a field name, becomes a projection function (function to access the field of a record or tuple); also, prefixes a
string literal A string literal (computer programming), literal or anonymous string is a String (computer science), string value in the source code of a computer program. Modern Computer programming, programming languages commonly use a quoted sequence of charact ...
to turn it into a character literal. * In
Mathematica Wolfram Mathematica is a software system with built-in libraries for several areas of technical computing that allow machine learning, statistics, symbolic computation, data manipulation, network analysis, time series analysis, NLP, optimizat ...
syntax, , when used as a variable, becomes a pure function (a placeholder that is mapped to any variable meeting the conditions). * In
LaTeX Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latexes are found in nature, but synthetic latexes are common as well. In nature, latex is found as a milky fluid found in 10% of all flowering plants (angiosperms ...
, , when prefixing a number, references an arguments for a user defined command. For instance \newcommand /code>. * In
Javadoc Javadoc (originally cased JavaDoc) is a documentation generator created by Sun Microsystems for the Java language (now owned by Oracle Corporation) for generating API documentation in HTML format from Java source code. The HTML format is used for ...
, is used with the tag to introduce or separate a field, constructor, or method member from its containing class. * In Redcode and some other dialects of
assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
, is used to denote immediate mode addressing, e.g., , which means "load accumulator A with the value 10" in
MOS 6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small te ...
assembly language. * in
HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScri ...
,
CSS Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS is a cornerstone techno ...
, SVG, and other computing applications is used to identify a color specified in
hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexa ...
format, e.g., . This usage comes from
X11 The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems. X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting wi ...
color specifications, which inherited it from early assembler dialects that used to prefix hexadecimal constants, e.g.: ZX Spectrum
Z80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples were ...
assembly. * In Be-Music Script, every command line starts with . Lines starting with characters other than "#" are treated as comments. * The use of the hash symbol in a
hashtag A hashtag is a metadata tag that is prefaced by the hash (also known as pound or octothorpe) sign, ''#''. On social media, hashtags are used on microblogging and photo-sharing services such as Twitter or Instagram as a form of user-generated ...
is a phenomenon conceived by
Chris Messina Christopher Messina (born August 11, 1974)Hochberg, Mina.Chris Messina stars in 'Ruby Sparks'. ''Newsday'' (Long Island, New York). August 8, 2012: “...the actor, who turns 38 Saturday ugust 11, 2012 After graduating from Northport ew Yor ...
, and popularized by social media network
Twitter Twitter is an online social media and social networking service owned and operated by American company Twitter, Inc., on which users post and interact with 280-character-long messages known as "tweets". Registered users can post, like, and ...
, as a way to direct conversations and topics amongst users. This has led to an increasingly common tendency to refer to the symbol itself as "hashtag". * In programming languages like PL/1 and Assembler used on IBM mainframe systems, as well as JCL (Job Control Language), the (along with and ) are used as additional letters in identifiers, labels and data set names. * In J, is the ''Tally'' or ''Count'' function, and similarly in
Lua Lua or LUA may refer to: Science and technology * Lua (programming language) * Latvia University of Agriculture * Last universal ancestor, in evolution Ethnicity and language * Lua people, of Laos * Lawa people, of Thailand sometimes referred t ...
, can be used as a shortcut to get the length of a table, or get the length of a string. Due to the ease of writing "#" over longer function names, this practice has become standard in the Lua community. * In Dyalog APL, is a reference to the root
namespace In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified. Namespaces ...
while is a reference to the current space's parent namespace.


Other uses

* Algebraic notation for chess: A hash after a move denotes
checkmate Checkmate (often shortened to mate) is any game position in chess and other chess-like games in which a player's king is in check (threatened with ) and there is no possible escape. Checkmating the opponent wins the game. In chess, the king is ...
. *
American Sign Language American Sign Language (ASL) is a natural language that serves as the predominant sign language of Deaf communities in the United States of America and most of Anglophone Canadians, Anglophone Canada. ASL is a complete and organized visual lang ...
transcription: The hash prefixing an all-caps word identifies a lexicalized fingerspelled sign, having some sort of blends or letter drops. All-caps words without the prefix are used for standard English words that are fingerspelled in their entirety. * Copy writing and
copy editing Copy editing (also known as copyediting and manuscript editing) is the process of revising written material ( copy) to improve readability and fitness, as well as ensuring that text is free of grammatical and factual errors. ''The Chicago Manual o ...
: Technical writers in
press release A press release is an official statement delivered to members of the news media for the purpose of providing information, creating an official statement, or making an announcement directed for public release. Press releases are also considere ...
s often use three number signs, directly above the boilerplate or underneath the body copy, indicating to media that there is no further copy to come. *
Footnote A note is a string of text placed at the bottom of a page in a book or document or at the end of a chapter, volume, or the whole text. The note can provide an author's comments on the main text or citations of a reference work in support of th ...
symbols (or endnote symbols): Due to ready availability in many fonts and directly on computer keyboards, "#" and other symbols (such as the
caret Caret is the name used familiarly for the character , provided on most QWERTY keyboards by typing . The symbol has a variety of uses in programming and mathematics. The name "caret" arose from its visual similarity to the original proofreade ...
) have in recent years begun to be occasionally used in catalogues and reports in place of more traditional symbols (esp. dagger, double-dagger,
pilcrow The pilcrow, ¶, is a handwritten or typographical character used to identify a paragraph. It is also called the paragraph mark (or sign or symbol), paraph, or blind P. The pilcrow may be used at the start of separate paragraphs or t ...
). * Linguistic
phonology Phonology is the branch of linguistics that studies how languages or dialects systematically organize their sounds or, for sign languages, their constituent parts of signs. The term can also refer specifically to the sound or sign system of a ...
: denotes a word boundary. For instance, means that becomes when it is the last segment in a word (i.e. when it appears before a word boundary). * Linguistic
syntax In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituency) ...
: A hash before an example sentence denotes that the sentence is semantically ill-formed, though grammatically well-formed. For instance, "#The toothbrush is pregnant" is a grammatically correct sentence, but the meaning is odd. *
Medical prescription A prescription, often abbreviated or Rx, is a formal communication from a physician or other registered health-care professional to a pharmacist, authorizing them to dispense a specific prescription drug for a specific patient. Historica ...
drug delimiter: In some countries, such as
Norway Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...
or
Poland Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populous ...
, is used as a delimiter between different drugs on medical prescriptions. * Medical shorthand: The hash is often used to indicate a
bone fracture A bone fracture (abbreviated FRX or Fx, Fx, or #) is a medical condition in which there is a partial or complete break in the continuity of any bone in the body. In more severe cases, the bone may be broken into several fragments, known as a '' ...
. For example, "#NOF" is often used for "fractured
neck of femur The femoral neck (femur neck or neck of the femur) is a flattened pyramidal process of bone, connecting the femoral head with the femoral shaft, and forming with the latter a wide angle opening medialward. Structure The neck is flattened from ...
". In radiotherapy, a full dose of radiation is divided into smaller doses or 'fractions'. These are given the shorthand to denote either the number of treatments in a prescription (e.g. 60Gy in 30#), or the fraction number (#9 of 25). *
Press release A press release is an official statement delivered to members of the news media for the purpose of providing information, creating an official statement, or making an announcement directed for public release. Press releases are also considere ...
s: The notation denotes "end", i.e. that there is no further copy to come. * As a
proofreading Proofreading is the reading of a galley proof or an electronic copy of a publication to find and correct reproduction errors of text or art. Proofreading is the final step in the editorial cycle before publication. Professional Traditional ...
mark, to indicate that a space should be inserted. * Publishing: When submitting a science fiction manuscript for publication, a number sign on a line by itself (indented or centered) indicates a
section break In books and documents, a section is a subdivision, especially of a chapter. Sections are visually separated from each other with a section break, typically consisting of extra space between the sections, and sometimes also by a section headin ...
in the text. *
Scrabble ''Scrabble'' is a word game in which two to four players score points by placing tiles, each bearing a single letter, onto a game board divided into a 15×15 grid of squares. The tiles must form words that, in crossword fashion, read left t ...
: Putting a number sign after a word indicates that the word is found in the British word lists, but not the North American lists. *
Teletext A British Ceefax football index page from October 2009, showing the three-digit page numbers for a variety of football news stories Teletext, or broadcast teletext, is a standard for displaying text and rudimentary graphics on suitably equipp ...
and
DVB Digital Video Broadcasting (DVB) is a set of international open standards for digital television. DVB standards are maintained by the DVB Project, an international industry consortium, and are published by a Joint Technical Committee (JTC) o ...
subtitles (in the UK and Ireland): The hash symbol, resembling music notation's sharp sign, is used to mark text that is either sung by a character or heard in background music, e.g.


Unicode

In Unicode, several # characters are assigned. Other attested names in Unicode are: pound sign, hash, crosshatch, octothorpe. At least three orthographically distinct number signs from other languages are also assigned: * * *


On keyboards

On the standard US keyboard layout, the symbol is . On standard UK and some other European keyboards, the same keystrokes produce the pound (sterling) sign, symbol, and may be moved to a separate key above the right shift key. If there is no key, the symbol can be produced on Windows with , on Mac OS with , and on Linux with .


Explanatory notes


References

{{navbox punctuation Typographical symbols