The symbol# is known variously in English-speaking regions as thenumber sign,^[1]hash,^[2] orpound sign.^[3] The symbol has historically been used for a wide range of purposes including the designation of an ordinal number and as aligatured abbreviation forpounds avoirdupois – having been derived from the now-rare℔.^[4]

Since 2007, widespread usage of the symbol to introducemetadata tags onsocial media platforms has led to such tags being known as "hashtags",^[5] and from that, the symbol itself is sometimes called ahashtag.^[6]

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

It is believed that the symbol traces its origins to the symbol℔,^[a] an abbreviation of the Roman termlibra pondo, which translates as "pound weight".^[7][8] The abbreviation "lb" was printed as a dedicatedligature including a horizontal line across (which indicated abbreviation).^[9][8] Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two slash-like strokes "//".^[8]

The symbol is described as the "number" character in an 1853 treatise onbookkeeping,^[10] and its double meaning is described in a bookkeeping text from 1880.^[11] The instruction manual of theBlickensderfer model 5 typewriter (c. 1896) appears to refer to the symbol as the "number mark".[12] Some early-20th-century U.S. sources refer to it as the "number sign",^[13] although this could also refer to thenumero sign (№).^[14] A 1917 manual distinguishes between two uses of the sign: "number (written before a figure)" and "pounds (written after a figure)".^[15] The use of the phrase "pound sign" to refer to this symbol is found from 1932 in U.S. usage.^[16] The termhash sign is found in South African writings from the late 1960s^[17] and from other non-North-American sources in the 1970s.^{[citation needed]}

For mechanical devices, the symbol appeared on the keyboard of theRemington Standard typewriter (c. 1886).[18] It appeared in many of the early teleprinter codes and from there was copied toASCII, 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 oftouch-tone keypads in 1968, but that button was not extensively used until the advent of large-scalevoicemail (PBX systems, etc.) in the early 1980s.^[4]

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.^[19] This usage inspiredChris Messina to propose a similar system to be used onTwitter to tag topics of interest on the microblogging network;^[20][21] this became known as ahashtag. Although used initially and most popularly on Twitter, hashtag use has extended to other social media sites.^[22]

Number sign

"Number sign" is the name chosen by theUnicode Consortium. Most common in Canada^[23] and the northeastern United States.^{[citation needed]} American telephone equipment companies which serve Canadian callers often have an option in their programming to denoteCanadian English, which in turn instructs the system to saynumber sign to callers instead ofpound.^[24]

Pound sign or pound

In the United States, the "#" key on a phone is commonly referred to as the pound sign,pound key, or simplypound. Dialing instructions to an extension such as #77, for example, can be read as "pound seven seven".^[25] This name is rarely used outside the United States, where the termpound sign is understood to meanthe currency symbol £.

Hash,hash mark,hashmark

In the United Kingdom,^[26] Australia,^[27] and some other countries,^{[citation needed]} it is generally called a "hash" (probably from "hatch", referring to cross-hatching^[28]).

Programmers also use this term; for instance#! 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 calledhashtag.^[6] Twitter documentation refers to it as "the hashtag symbol".^[29]

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.^[30][31]

Octothorp, octothorpe, octathorp, octatherp

Most scholars believe the word was invented by workers at theBell Telephone Laboratories by 1968,^[32] who needed a word for the symbol on thetelephone keypad. Don MacPherson is said to have created the word by combiningocto and the last name ofJim Thorpe, an Olympic medalist.^[33] Howard Eby and Lauren Asplund claim to have invented the word as a joke in 1964, combiningocto with the syllabletherp which, because of the "th"digraph, was hard to pronounce in different languages.^[34]The Merriam-Webster New Book of Word Histories, 1991, has a long article that is consistent with Doug Kerr's essay,^[34] 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 ofJames Oglethorpe^[35] or using the Old English word for village,thorp, because the symbol looks like a village surrounded by eight fields.^[36][37] The word was popularized within and outside Bell Labs.^[38] 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".^[39]

Sharp

Use of the name "sharp" is due to the symbol's resemblance toU+266F♯MUSIC SHARP SIGN. The same derivation is seen in the name of theMicrosoft programming languagesC#,J# andF#. Microsoft says that the nameC# is pronounced 'see sharp'."^[40] According to the ECMA-334 C# Language Specification, the name of the language is written "C#" ("LATIN CAPITAL LETTER C (U+0043) followed by theNUMBER SIGN # (U+0023)") and pronounced "C Sharp".^[41]

Square

On telephones, theInternational Telecommunication Union specificationITU-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."^[42] Formally, this is not a number sign but rather another character,U+2317⌗VIEWDATA SQUARE. The real or virtual keypads on almost all modern telephones use the simple# instead, as does most documentation.^{[citation needed]}

Other

Names that may be seen include:^{[4][43][better source needed]}crosshatch, crunch, fence, flash, garden fence, garden gate, gate, grid, hak, mesh, oof, pig-pen, punch mark, rake, score, scratch, scratch mark,tic-tac-toe, andunequal.

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. The use of⟨#⟩ as an abbreviation for "number" is common in informal writing, but use in print is rare.^[44] Where Americans might write "Symphony #5", British and Irish people usually write "Symphony No. 5".^{[citation needed]}

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. This usage is rare outside North America, where "lb" or "lbs" is used.

⟨#⟩ isnot a replacement for thepound sign⟨£⟩, but British typewriters and keyboards have a£ key where American keyboards have a# key.^[45] Many early computer and teleprinter codes (such asBS 4730 (the UK national variant of theISO/IEC 646 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 to avoid confusing dollars and pounds in financial communications).

• Inset theory, #S is one possible notation for thecardinality or size of thesetS, instead of${\displaystyle |S|}$ . That is, for a set${\displaystyle S=\{s_1,s_2,s_3,\dots ,s_n\}}$ , in which all${\displaystyle s_i}$  are mutually distinct,${\displaystyle \mathrm{\#}S=n=|S|.}$  This notation is only sometimes used forfinite sets, usually innumber theory, to avoid confusion with thedivisibility symbol, e.g.${\displaystyle a\mid b}$ .
• Intopology,A#B is theconnected sum ofmanifoldsA andB, or of knotsA andB inknot theory.
• Innumber theory,n# is theprimorial ofn.
• Inconstructive mathematics, # denotes anapartness relation.
• Incomputational complexity theory,#P denotes acomplexity class ofcounting problems. The standard notation for this class uses the number sign symbol, not thesharp 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.

• InUnicode andASCII, the symbol has acode point asU+0023#NUMBER SIGN and# inHTML5.^[46]
• 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.^[47] 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 (seemagic number). This combination was chosen so it would be a comment in the scripting languages.
  • #! is the symbol of the CrunchBang Linux distribution.
• In thePerl programming language,# is used as a modifier to array syntax to return the index number of the last element in thearray, e.g., an array's last element is at$array[$#array]. The number of elements in the array is$#array + 1, since Perl arrays default to using zero-based indices. If the array has not been defined, the return is also undefined. If the array is defined but has not had any elements assigned to it, e.g.,@array = (), then$#array returns−1. See the section onArray functions in the Perl language structure article.
• In both theC andC++ preprocessors, as well as in other syntactically C-like languages,# is used to start a preprocessordirective. Inside macros, after#define, it is used for various purposes; for example, the double pound (hash) sign## is used for tokenconcatenation.
• InUnix shells,# is placed by convention at the end of acommand prompt to denote that the user is working asroot.
• # is used in aURL of aweb page or other resource to introduce a "fragment identifier" – an id which defines a position within that resource. In HTML, this is known as ananchor link. For example, in the URLhttps://en.wikipedia.org/wiki/Number_sign#Computing the portion after the# (Computing) is the fragment identifier, in this case denoting that the display should be moved to show the tag marked by<spanid="Computing">...</span> in the HTML.^[48]
• Internet Relay Chat: on (IRC) servers,# precedes the name of everychannel that is available across an entire IRC network.
• Inblogs,# is sometimes used to denote apermalink for that particular weblog entry.
• Inlightweight markup languages, such aswikitext,# is often used to introduce numbered list items.
• # is used in theModula-2 andOberon programming languages designed byNiklaus Wirth and in theComponent Pascal language derived from Oberon to denote thenot equal symbol, as a stand-in for the mathematical unequal sign≠, being more intuitive than<> or!=. For example:IFi#0THEN...
• InRust,# is used for attributes such as in#[test].
• InOCaml,# is the operator used to call a method.
• InCommon Lisp,^[49]# is a dispatchingread macro character used to extend theS-expression syntax with short cuts and support for various data types (complex numbers, vectors and more).
• InScheme,# is the prefix for certain syntax with special meaning.
• InStandard ML,#, when prefixed to a field name, becomes a projection function (function to access the field of a record or tuple); also,# prefixes astring literal to turn it into a character literal.
• InMathematica syntax,#, when used as a variable, becomes a pure function (a placeholder that is mapped to any variable meeting the conditions).
• InLaTeX,#, when prefixing a number, references an arguments for a user defined command. For instance\newcommand{\code}[1]{\texttt{#1}}.
• InJavadoc,^[50]# is used with the@see tag to introduce or separate a field, constructor, or method member from its containing class.
• InRedcode and some other dialects ofassembly language,# is used to denote immediate mode addressing, e.g.,LDA #10, which means "load accumulator A with the value 10" inMOS 6502 assembly language.
• inHTML,CSS,SVG, and other computing applications# is used to identify a color specified inhexadecimal format, e.g.,#FFAA00. This usage comes fromX11 color specifications, which inherited it from early assembler dialects that used# to prefix hexadecimal constants, e.g.: ZX SpectrumZ80 assembly.^[51]
• InBe-Music Script, every command line starts with#. Lines starting with characters other than "#" are treated as comments.
• The use of the hash symbol in ahashtag is a phenomenon conceived byChris Messina, and popularized by social media networkTwitter, 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".^[52]
• 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.
• InJ,# is theTally orCount function,^[53] and similarly inLua,# 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 DyalogAPL,# is a reference to the rootnamespace while## is a reference to the current space's parent namespace.
• InAda, the# character is used in based integer literals, which take the formbase#digits#, wherebase is an integer from 2 to 16 specifying the radix, anddigits are the digits valid in that base (0-9, optionally A-F for bases above 10).

• Algebraic notation for chess: A hash after a move denotescheckmate.
• 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.^[54]
• Copy writing andcopy editing: Technical writers inpress releases 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.^[55]
• Footnote symbols (or endnote symbols): Due to ready availability in many fonts and directly on computer keyboards, "#" and other symbols (such as thecaret) have in recent years begun to be occasionally used in catalogues and reports in place of more traditional symbols (esp.dagger, double-dagger,pilcrow).
• Linguisticphonology:# denotes a word boundary. For instance,/d/ → [t] / _# means that/d/ becomes[t] when it is the last segment in a word (i.e. when it appears before a word boundary).
• Linguisticsyntax: 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.^[56][57]
• Medical prescription drug delimiter: In some countries, such asNorway orPoland,# is used as adelimiter between different drugs on medical prescriptions.
• Medical shorthand: The hash is often used to indicate abone fracture.^[58] For example, "#NOF" is often used for "fracturedneck of femur". 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).
• As aproofreading mark, to indicate that a space should be inserted.^[59]
• Publishing: When submitting a science fiction manuscript for publication, a number sign on a line by itself (indented or centered) indicates asection break in the text.^[60]
• Scrabble: Putting a number sign after a word indicates that the word is found in the British word lists, but not theNorth American lists.^[61]
• Teletext andDVBsubtitles (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.# For he's a jolly good fellow #

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

Unicode characters with "number sign" in their names:

• U+0023#NUMBER SIGN Otherattested names in Unicode are:POUND SIGN, HASH, CROSSHATCH, OCTOTHORPE.
• U+0600؀ARABIC NUMBER SIGN
• U+0BFA௺TAMIL NUMBER SIGN
• U+1AC6᫆COMBINING NUMBER SIGN ABOVE
• U+FE5F﹟SMALL NUMBER SIGN
• U+FF03＃FULLWIDTH NUMBER SIGN
• U+110BDKAITHI NUMBER SIGN
• U+110CD𑃍KAITHI NUMBER SIGN ABOVE
• U+11FE9𑿩TAMIL TRADITIONAL NUMBER SIGN
• U+E0023TAG NUMBER SIGN

Additionally, a Unicode named sequenceKEYCAP NUMBER SIGN is defined for thegrapheme clusterU+0023+FE0F+20E3 (#️⃣).^[62]

On the standardUS keyboard layout, the# symbol is⇧ Shift+3. On standard UK and some other European keyboards, the same keystrokes produce thepound (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 withAlt+35, on Mac OS with⌥ Opt+3, and on Linux withCompose++.

• pound sign,£
• sharp sign,♯
• viewdata square,⌗,
• equal and parallel to symbol,⋕
• looped square,⌘
• the Chinese character for "well"井
• the gametic-tac-toe