Number sign

Template:Short description Template:Distinguish Template:Infobox symbol

The symbol Template:Char is known as the number sign,<ref name="oedNumberSign">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> hash,<ref name="oedHash">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> or pound sign<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> (only "hash" is used outside North America). 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 Template:Char.<ref name="Houston">Template:Cite book</ref>

Since 2007, widespread usage of the symbol to introduce metadata tags on social media platforms has led to such tags being known as "hashtags",<ref>Template:Cite book</ref> and from that, the symbol itself is sometimes called a hashtag.<ref name=hashtag>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The symbol is distinguished from similar symbols by its combination of level horizontal strokes and right-tilting vertical strokes.

HistoryEdit

File:Pfund.svg

A stylized version of the abbreviation for libra pondo ("pound weight")

File:Libra pondo abbreviation newton.jpg

The abbreviation written by Isaac Newton, showing the evolution from "Template:Not a typo" toward "#"

It is believed that the symbol traces its origins to the symbol Template:Angbr,Template:Efn an abbreviation of the Roman term libra pondo, which translates as "pound weight".<ref>Template:Cite book</ref><ref name="NewYorker" /> The abbreviation "lb" was printed as a dedicated ligature including a horizontal line across (which indicated abbreviation).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="NewYorker" /> Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two slash-like strokes "//".<ref name="NewYorker">Template:Cite news</ref>

The symbol is described as the "number" character in an 1853 treatise on bookkeeping,<ref>Template:Cite book</ref> and its double meaning is described in a bookkeeping text from 1880.<ref>Template:Cite book</ref> The instruction manual of the Blickensderfer model 5 typewriter (Template:Circa) appears to refer to the symbol as the "number mark".<ref>Template:Cite book</ref> Some early-20th-century U.S. sources refer to it as the "number sign",<ref>e.g. J. W. Marley, "The Detection and Illustration of Forgery By Comparison of Handwriting", in Template:Cite book</ref> although this could also refer to the numero sign (Template:Not a typo).<ref>e.g. The British Printer vol. viii (1895), p. 395</ref> A 1917 manual distinguishes between two uses of the sign: "number (written before a figure)" and "pounds (written after a figure)".<ref>Template:Cite book</ref> The use of the phrase "pound sign" to refer to this symbol is found from 1932 in U.S. usage.<ref>Template:Cite book</ref> The term hash sign is found in South African writings from the late 1960s<ref>Research Review. Navorsingsoorsig vols. 18–21, pp. 117, 259 (1968)</ref> and from other non-North-American sources in the 1970s.Template:Citation needed

For mechanical devices, the symbol appeared on the keyboard of the Remington Standard typewriter (Template:Circa).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It appeared in many of the early teleprinter codes and from there was copied to ASCII, 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 keypads in 1968, but that button was not extensively used until the advent of large-scale voicemail (PBX systems, etc.) in the early 1980s.<ref name="Houston" />

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) networks circa 1988 to label groups and topics.<ref>"Channel Scope". Section 2.2. Template:IETF RFC</ref> This usage inspired Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the microblogging network;<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite news</ref> this became known as a hashtag. Although used initially and most popularly on Twitter, hashtag use has extended to other social media sites.<ref>Template:Cite news</ref> Template:Anchor

NamesEdit

Number sign

"Number sign" is the name chosen by the Unicode Consortium. Most common in Canada<ref>Template:Cite book</ref> and the northeastern United States.Template:Citation needed American telephone equipment companies which serve Canadian callers often have an option in their programming to denote Canadian English, which in turn instructs the system to say number sign to callers instead of pound.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> This name is rarely used elsewhere in the world, where numbers are normally represented by the letters "No.". Pound sign or pound

In the United States and Canada, the "#" key on a phone is commonly referred to as the pound sign, pound key, or simply pound. Dialing instructions to an extension such as #77, for example, can be read as "pound seven seven".<ref name="nyt">Template:Cite news</ref> This name is rarely used elsewhere, as the term pound sign is understood to mean the currency symbol £.

Hash, hash mark, hashmark

In the United Kingdom,<ref>Template:Cite news</ref> Australia,<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> and some other countries,Template:Citation needed it is frequently called a "hash" (probably from "hatch", referring to cross-hatching<ref>Template:Cite dictionary</ref>).

Programmers also use this term; for instance <syntaxhighlight lang="text" class="" style="" inline="1">#!</syntaxhighlight> is "hash, bang" or "shebang".

Hashtag

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.<ref name=hashtag /> Twitter documentation refers to it as "the hashtag symbol".<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> 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.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Template:Vanchor, octothorpe, octathorp, octatherp

Most scholars believe the word was invented by workers at the Bell Telephone Laboratories by 1968,<ref>Template:Cite magazine</ref> who needed a word for the symbol on the telephone keypad. Don MacPherson is said to have created the word by combining octo and the last name of Jim Thorpe, an Olympic medalist.<ref>Ralph Carlsen, "What the ####?" Telecoms Heritage Journal 28 (1996): 52–53.</ref> 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.<ref name="Kerr">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> The Merriam-Webster New Book of Word Histories, 1991, has a long article that is consistent with Doug Kerr's essay,<ref name="Kerr"/> 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<ref>John Baugh, Robert Hass, Maxine H. Kingston, et al., "Octothorpe", The American Heritage Dictionary of the English Language (Boston: Houghton Mifflin, 2000)</ref> or using the Old English word for village, thorp, because the symbol looks like a village surrounded by eight fields.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Bringhurst, "Octothorpe". Elements of Typographic Style</ref> The word was popularized within and outside Bell Labs.<ref>"You Asked Us: About the * and # on the New Phones", The Calgary Herald, September 9, 1972, 90.</ref> 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".<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Sharp

Use of the name "sharp" is due to the symbol's resemblance to Template:Unichar. The same derivation is seen in the name of the Microsoft programming languages C#, J# and F#. Microsoft says that the name C# is pronounced 'see sharp'."<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> According to the ECMA-334 C# Language Specification, the name of the language is written "C#" ("Template:Resize (U+0043) followed by the Template:Resize # (U+0023)") and pronounced "C Sharp".<ref>Template:Cite book</ref> Square

File:Detail-Tastatur-FeTAp-751-1982.JPG

Detail of a telephone keypad displaying the Viewdata square

On telephones, the International Telecommunication Union 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."<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> Formally, this is not a number sign but rather another character, Template:Unichar. The real or virtual keypads on almost all modern telephones use the simple <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> instead, as does most documentation.Template:Cn

UsageEdit

When Template:Angbr prefixes a number, it is read as "number". "A #2 pencil", for example, indicates "a number-two pencil". This usage is rare in print,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> or outside North America. Instead the abbreviation Template:Notatypo or the symbol Template:Notatypo or just "number" is used.Template:Cn

When Template:Angbr is after a number, it is read as "pound" or "pounds", meaning the unit of weight.Template:Cn The text "5# bag of flour" would mean "five-pound bag of flour". Again this is rare outside North America.

MathematicsEdit

In set theory, #S is one possible notation for the cardinality or size of the set S, instead of <math>|S|</math>. That is, for a set <math>S = \{s_1,s_2,s_3, \dots , s_n\}</math>, in which all <math>s_i</math> are mutually distinct, <math>\#S = n = |S|.</math> This notation is only sometimes used for finite sets, usually in number theory, to avoid confusion with the divisibility symbol, e.g. <math>a \mid b</math>.
In topology, A#B is the connected sum of manifolds A and B, or of knots A and B in knot theory.
In number theory, n# is the primorial of n.
In constructive mathematics, # denotes an apartness relation.
In computational complexity theory, #P denotes a complexity class 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.

ComputingEdit

In Unicode and ASCII, the symbol has a code point as Template:Unichar and entity code <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> in HTML5.<ref>HTML5 is the only version of HTML that has a named entity for the number sign, see https://www.w3.org/TR/html4/sgml/entities.html Template:Webarchive ("The following sections present the complete lists of character entity references.") and https://www.w3.org/TR/2014/CR-html5-20140731/syntax.html#named-character-references Template:Webarchive ("num;").</ref>
In many scripting languages and data file formats, especially ones that originated on Unix, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> introduces a comment that goes to the end of the line.<ref name="hash character">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> The combination <syntaxhighlight lang="text" class="" style="" inline="1">#!</syntaxhighlight> 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.

- <syntaxhighlight lang="text" class="" style="" inline="1">#!</syntaxhighlight> is the symbol of the CrunchBang Linux distribution.
In the Perl programming language, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is used as a modifier to array syntax to return the index number of the last element in the array, e.g., an array's last element is at <syntaxhighlight lang="text" class="" style="" inline="1">$array[$#array]</syntaxhighlight>. The number of elements in the array is <syntaxhighlight lang="text" class="" style="" inline="1">$#array + 1</syntaxhighlight>, since Perl arrays default to using zero-based indices. If the array has not been defined, the return is also undefined. If the array is defined but has not had any elements assigned to it, e.g., <syntaxhighlight lang="text" class="" style="" inline="1">@array = ()</syntaxhighlight>, then <syntaxhighlight lang="text" class="" style="" inline="1">$#array</syntaxhighlight> returns <syntaxhighlight lang="text" class="" style="" inline="1">−1</syntaxhighlight>. See the section on Array functions in the Perl language structure article.
In both the C and C++ preprocessors, as well as in other syntactically C-like languages, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is used to start a preprocessor directive. Inside macros, after <syntaxhighlight lang="text" class="" style="" inline="1">#define</syntaxhighlight>, it is used for various purposes; for example, the double pound (hash) sign <syntaxhighlight lang="text" class="" style="" inline="1">##</syntaxhighlight> is used for token concatenation.
In Unix shells, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is placed by convention at the end of a command prompt to denote that the user is working as root.
<syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is used in a URL of a web page or other resource to introduce a "fragment identifier" – an id which defines a position within that resource. In HTML, this is known as an anchor link. For example, in the URL <syntaxhighlight lang="text" class="" style="" inline="1">https://en.wikipedia.org/wiki/Number_sign#Computing</syntaxhighlight> the portion after the <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> (<syntaxhighlight lang="text" class="" style="" inline="1">Computing</syntaxhighlight>) is the fragment identifier, in this case denoting that the display should be moved to show the tag marked by <syntaxhighlight lang="html" class="" style="" inline="1">...</syntaxhighlight> in the HTML.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

Internet Relay Chat: on (IRC) servers, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> precedes the name of every channel that is available across an entire IRC network.
In blogs, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is sometimes used to denote a permalink for that particular weblog entry.
In lightweight markup languages, such as wikitext, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is often used to introduce numbered list items.
<syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is used in the Modula-2 and Oberon programming languages designed by Niklaus Wirth and in the Component Pascal language derived from Oberon to denote the not equal symbol, as a stand-in for the mathematical unequal sign Template:Char, being more intuitive than <syntaxhighlight lang="text" class="" style="" inline="1"><></syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1">!=</syntaxhighlight>. For example: Template:Nowrap
In Rust, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is used for attributes such as in <syntaxhighlight lang="text" class="" style="" inline="1">#[test]</syntaxhighlight>.
In OCaml, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is the operator used to call a method.
In Common Lisp,<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is a dispatching read macro character used to extend the S-expression syntax with short cuts and support for various data types (complex numbers, vectors and more).

In Scheme, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is the prefix for certain syntax with special meaning.
In Standard ML, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight>, when prefixed to a field name, becomes a projection function (function to access the field of a record or tuple); also, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> prefixes a string literal to turn it into a character literal.
In Mathematica syntax, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight>, when used as a variable, becomes a pure function (a placeholder that is mapped to any variable meeting the conditions).
In LaTeX, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight>, when prefixing a number, references an arguments for a user defined command. For instance <syntaxhighlight lang="tex" inline>\newcommand{\code}[1]{\texttt{#1}}</syntaxhighlight>.
In Javadoc,<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is used with the <syntaxhighlight lang="text" class="" style="" inline="1">@see</syntaxhighlight> tag to introduce or separate a field, constructor, or method member from its containing class.

In Redcode and some other dialects of assembly language, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is used to denote immediate mode addressing, e.g., <syntaxhighlight lang="text" class="" style="" inline="1">LDA #10</syntaxhighlight>, which means "load accumulator A with the value 10" in MOS 6502 assembly language.
in HTML, CSS, SVG, and other computing applications <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is used to identify a color specified in hexadecimal format, e.g., <syntaxhighlight lang="text" class="" style="" inline="1">#FFAA00</syntaxhighlight>. This usage comes from X11 color specifications, which inherited it from early assembler dialects that used <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> to prefix hexadecimal constants, e.g.: ZX Spectrum Z80 assembly.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

In Be-Music Script, every command line starts with <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight>. Lines starting with characters other than "#" are treated as comments.
The use of the hash symbol in a hashtag is a phenomenon conceived by Chris Messina, and popularized by social media network Twitter, 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".<ref name="Time">Template:Cite magazine</ref>
In programming languages like PL/1 and Assembler used on IBM mainframe systems, as well as JCL (Job Control Language), the <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> (along with <syntaxhighlight lang="text" class="" style="" inline="1">$</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1">@</syntaxhighlight>) are used as additional letters in identifiers, labels and data set names.
In J, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is the Tally or Count function,<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> and similarly in Lua, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> 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, <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> is a reference to the root namespace while <syntaxhighlight lang="text" class="" style="" inline="1">##</syntaxhighlight> is a reference to the current space's parent namespace.
In Ada, the <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> character is used in based integer literals, which take the form <syntaxhighlight lang="text" class="" style="" inline="1">base#digits#</syntaxhighlight>, where <syntaxhighlight lang="text" class="" style="" inline="1">base</syntaxhighlight> is an integer from 2 to 16 specifying the radix, and <syntaxhighlight lang="text" class="" style="" inline="1">digits</syntaxhighlight> are the digits valid in that base (0-9, optionally A-F for bases above 10).

Other usesEdit

Algebraic notation for chess: A hash after a move denotes checkmate.
American Sign Language 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.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

Copy writing and copy editing: Technical writers in press releases often use three number signs, Template:Char directly above the boilerplate or underneath the body copy, indicating to media that there is no further copy to come.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

Footnote symbols (or endnote symbols): Due to ready availability in many fonts and directly on computer keyboards, "#" and other symbols (such as the caret) have in recent years begun to be occasionally used in catalogues and reports in place of more traditional symbols (esp. dagger, double-dagger, pilcrow).
Linguistic phonology: Template:Char denotes a word boundary. For instance, <syntaxhighlight lang="text" class="" style="" inline="1">/d/ → [t] / _#</syntaxhighlight> means that Template:Char becomes Template:Char when it is the last segment in a word (i.e. when it appears before a word boundary).
Linguistic syntax: 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.<ref>Template:Cite book</ref><ref>Template:Cite book</ref>
Medical prescription drug delimiter: In some countries, such as Norway or Poland, Template:Char is used as a delimiter between different drugs on medical prescriptions.
Medical shorthand: The hash is often used to indicate a bone fracture.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> For example, "#NOF" is often used for "fractured neck of femur". In radiotherapy, a full dose of radiation is divided into smaller doses or 'fractions'. These are given the shorthand Template:Char to denote either the number of treatments in a prescription (e.g. 60Gy in 30#), or the fraction number (#9 of 25).

As a proofreading mark, to indicate that a space should be inserted.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }} from Merriam Webster</ref>

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 the text.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

Scrabble: Putting a number sign after a word indicates that the word is found in the British word lists, but not the North American lists.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

Teletext and DVB 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. Template:Mono

UnicodeEdit

The number sign was assigned code 35 (hex 0x23) in ASCII where it was inherited by many character sets. In EBCDIC it is often at 0x7B or 0xEC.

Unicode characters with "number sign" in their names:

Template:Unichar (Other attested names in Unicode are: Template:Sc.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>)

Additionally, a Unicode named sequence Template:Resize is defined for the grapheme cluster <syntaxhighlight lang="text" class="" style="" inline="1">U+0023+FE0F+20E3</syntaxhighlight> (#️⃣).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>Template:Efn

On keyboardsEdit

On the standard US keyboard layout, the Template:Char symbol is Template:Keypress. On standard UK and some other European keyboards, the same keystrokes produce the [[Pound sign|pound (sterling) sign, Template:Char symbol]], and Template:Keypress may be moved to a separate key above the right shift key.