Number sign#Other uses

File:Pfund.svg ("pound weight")|upright]]

File:Libra pondo abbreviation newton.jpg, showing the evolution from "{{not a typo|℔}}" toward "#"]]

It is believed that the symbol traces its origins to the symbol {{char|℔}},{{efn|{{unichar|2114|html=|nlink=}} }} an abbreviation of the Roman term libra pondo, which translates as "pound weight".{{cite book | quote=The Italian libbra (from the old Latin word libra, 'balance') represented a weight almost exactly equal to the avoirdupois pound of England. The Italian abbreviation of lb with a line drawn across the letters was used for both weights. | author =Keith Gordon Irwin | title= The romance of writing, from Egyptian hieroglyphics to modern letters, numbers, and signs | page= 125 | location= New York |publisher= Viking Press | year = 1967 | orig-year = 1956}} The abbreviation "lb" was printed as a dedicated ligature including a horizontal line across (which indicated abbreviation).{{cite web | url= http://www.royalmintmuseum.org.uk/Default.aspx?PageID=14314417 | title=The Origins of £sd | publisher=The Royal Mint Museum |archive-url = https://web.archive.org/web/20200308111135/http://www.royalmintmuseum.org.uk/Default.aspx?PageID=14314417 |archive-date= 8 March 2020 |quote=It is not known for certain when the horizontal line or lines, which indicate an abbreviation, first came to be drawn through the L.}} Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two slash-like strokes "//".{{cite news|last=Houston|first=Keith|title=The Ancient Roots of Punctuation|url=http://www.newyorker.com/online/blogs/books/2013/09/origins-of-hashtag-manicule-diple-pilcrow-ampersand-explained.html|access-date=16 October 2013|newspaper=The New Yorker|date=2013-09-06|archive-date=2014-06-25|archive-url=https://web.archive.org/web/20140625175614/http://www.newyorker.com/online/blogs/books/2013/09/origins-of-hashtag-manicule-diple-pilcrow-ampersand-explained.html|url-status=live}}

The symbol is described as the "number" character in an 1853 treatise on bookkeeping,{{cite book|last1=Crittendon|first1=S. W.|title=An Elementary Treatise on Book-keeping by Single and Double Entry |date=1853 |publisher=E., C., & J. Biddle |location=Philadelphia |page=10 |url=https://archive.org/details/anelementarytre00critgoog/page/n16/mode/1up |access-date=7 February 2023}} and its double meaning is described in a bookkeeping text from 1880.{{cite book|last1=Duff|first1=C. P.|last2=Duff|first2=W. H.|last3=Duff|first3=R. P.|title=Book-Keeping By Single and Double Entry|date=1880|publisher=Harper and Brothers|page=[https://archive.org/details/bookkeepingbysi00duffgoog/page/n27 21]|url=https://archive.org/details/bookkeepingbysi00duffgoog|access-date= 24 November 2015}} The instruction manual of the Blickensderfer model 5 typewriter ({{Circa|1896}}) appears to refer to the symbol as the "number mark".{{cite book |author= |url=http://site.xavier.edu/polt/typewriters/Blick5manual.pdf |title=Method of Operating and Instructions for Practice on the Blickensderfer Typewriter |publisher=K. M. Turner |year=1896 |location=Atlanta, GA |page=14 |quote=It is best to use the 'number mark' for plus; the hyphen for minus, and two hyphens for the sign = |archive-url=https://web.archive.org/web/20211014223637/https://site.xavier.edu/polt/typewriters/Blick5manual.pdf |archive-date=Oct 14, 2021}} Some early-20th-century U.S. sources refer to it as the "number sign",e.g. J. W. Marley, "The Detection and Illustration of Forgery By Comparison of Handwriting", in {{cite book |url=https://books.google.com/books?id=Z_NEAQAAMAAJ&pg=PA180 |title=Proceedings of the Sixteenth Annual Convention of the Kansas Bankers' Association |publisher=Rusell |year=1903 |location=Kansas City |page=180}} although this could also refer to the numero sign ({{not a typo|№}}).e.g. [https://books.google.com/books?id=CPodAQAAIAAJ&pg=RA2-PA395 The British Printer vol. viii] (1895), p. 395 A 1917 manual distinguishes between two uses of the sign: "number (written before a figure)" and "pounds (written after a figure)".{{cite book |title=Business Arithmetic for Secondary Schools |first=Ernest L. |last=Thurston |location=New York |publisher=Macmillan |year=1917 |page=[https://archive.org/details/businessarithme02thurgoog/page/n439 419] |url=https://archive.org/details/businessarithme02thurgoog|quote=business symbols pound. }} The use of the phrase "pound sign" to refer to this symbol is found from 1932 in U.S. usage.{{cite book |first=Nancy M. |last=Lawrence |author2=F. Ethel McAfee |author3=Mildred M. Butler |title=Correlated studies in stenography |publisher=Gregg |year=1932 |page=141 |url=https://books.google.com/books?id=oCsOAQAAIAAJ}} The term hash sign is found in South African writings from the late 1960sResearch Review. Navorsingsoorsig vols. 18–21, pp. 117, 259 (1968) and from other non-North-American sources in the 1970s.{{citation needed|date=November 2017}}

For mechanical devices, the symbol appeared on the keyboard of the Remington Standard typewriter ({{Circa|1886}}).{{cite web|url=http://pds.lib.harvard.edu/pds/view/2574435?n=52&s=4&printThumbnails=no|title=Remington Standard typewriter |location=New York |publisher=Wyckoff, Seamans & Benedict |date=1886 |page=50}} 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.

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."Channel Scope". Section 2.2. {{IETF RFC|2811}} This usage inspired Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the microblogging network;{{cite web|url=http://www.cmu.edu/homepage/computing/2014/summer/originstory.shtml|title=#OriginStory|publisher=Carnegie Mellon University|date=August 29, 2014|access-date=August 23, 2019|archive-date=June 1, 2019|archive-url=https://web.archive.org/web/20190601233044/https://www.cmu.edu/homepage/computing/2014/summer/originstory.shtml|url-status=live}}{{cite news |author=Parker, Ashley |date=June 10, 2011 |title=Twitter's Secret Handshake |work=The New York Times |url=https://www.nytimes.com/2011/06/12/fashion/hashtags-a-new-way-for-tweets-cultural-studies.html?_r=1&pagewanted=all |url-status=live |url-access=limited |access-date=July 26, 2011 |archive-url=https://web.archive.org/web/20110617001914/https://www.nytimes.com/2011/06/12/fashion/hashtags-a-new-way-for-tweets-cultural-studies.html?_r=1&pagewanted=all |archive-date=Jun 17, 2011}} this became known as a hashtag. Although used initially and most popularly on Twitter, hashtag use has extended to other social media sites.{{cite news |last=Warren |first=Christina |title=Facebook finally gets #hashtags |website=CNN |url=http://www.cnn.com/2013/06/12/tech/social-media/facebook-hashtags/index.html |url-status=live |access-date=July 16, 2006 |archive-url=https://web.archive.org/web/20130613102217/https://www.cnn.com/2013/06/12/tech/social-media/facebook-hashtags/index.html |archive-date=Jun 13, 2013}}

{{anchor|Other_names_in_English}}

Number sign

:"Number sign" is the name chosen by the Unicode Consortium. Most common in Canada{{cite book |editor-last=Barber |editor-first=Katherine |title=The Canadian Oxford dictionary |date=2004 |publisher=Oxford University Press |location=Toronto |isbn=0195418166 |edition=2nd}} and the northeastern United States.{{citation needed|date=October 2020}} 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.{{cite web |title=Norstar Voice Mail 4.1 {{!}} Software Add-on Guide |publisher=Nortel |page=12 |url=https://downloads.avaya.com/css/P8/documents/100141949 |access-date=2015-12-11 |archive-date=2015-12-22 |archive-url=https://web.archive.org/web/20151222090552/https://downloads.avaya.com/css/P8/documents/100141949 |url-status=live }} 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".{{cite news | url=https://www.nytimes.com/1991/03/24/magazine/on-language-hit-the-pound-sign.html | title=On Language; Hit the Pound Sign | author=William Safire | newspaper=The New York Times | access-date=May 21, 2011 | date=March 24, 1991 | author-link=William Safire | archive-date=July 21, 2010 | archive-url=https://web.archive.org/web/20100721065416/http://www.nytimes.com/1991/03/24/magazine/on-language-hit-the-pound-sign.html | url-status=live }} 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,{{cite news |url=https://www.theguardian.com/artanddesign/2010/dec/08/hash-symbol-twitter-typography |title=How the # became the sign of our times |newspaper=The Guardian |access-date=30 December 2014 |archive-date=31 December 2014 |archive-url=https://web.archive.org/web/20141231061916/http://www.theguardian.com/artanddesign/2010/dec/08/hash-symbol-twitter-typography |url-status=live }} Australia,{{cite web |url=https://getproofed.com.au/writing-tips/writing-tips-how-to-use-the-hash-sign/ |title=Writing Tips: How to Use the Hash Sign (#) |website=GetProofed |date=6 February 2020 |quote=In Australia, however, it was better known as the 'hash' sign and only used to mean 'number'. |access-date=9 January 2023 |archive-date=9 January 2023 |archive-url=https://web.archive.org/web/20230109105954/https://getproofed.com.au/writing-tips/writing-tips-how-to-use-the-hash-sign/ |url-status=live }} and some other countries,{{citation needed|date=November 2021}} it is frequently called a "hash" (probably from "hatch", referring to cross-hatching{{cite dictionary |url=http://www.oed.com/view/Entry/84445 |title=Hash sign |dictionary=Oxford English Dictionary |access-date=14 October 2013 |archive-date=16 November 2018 |archive-url=https://web.archive.org/web/20181116124751/http://www.oed.com/view/Entry/84445 |url-status=live }}).

:Programmers also use this term; for instance {{code|#!}} 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. Twitter documentation refers to it as "the hashtag symbol".{{cite web|url=https://support.twitter.com/articles/49309|title=Using hashtags on Twitter|website=Twitter|access-date=5 May 2016|archive-date=4 May 2016|archive-url=https://web.archive.org/web/20160504192917/https://support.twitter.com/articles/49309|url-status=live}}

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.{{cite web|title=A Dictionary of Singlish and Singapore English|author=Jack Tsen-Ta Lee|url=http://mysmu.edu/faculty/jacklee/singlish_H.htm#hex|access-date=14 January 2016|archive-date=19 January 2016|archive-url=https://web.archive.org/web/20160119143130/http://www.mysmu.edu/faculty/jacklee/singlish_H.htm#hex|url-status=live}}{{cite web|title=Address Formats|url=http://www.informatica.com/products/data-quality/data-as-a-service/address-verification/address-formats.html?code=SGP|access-date=14 January 2016|archive-date=8 March 2016|archive-url=https://web.archive.org/web/20160308064622/https://www.informatica.com/products/data-quality/data-as-a-service/address-verification/address-formats.html?code=SGP|url-status=live}}

{{vanchor|Octothorp}}, octothorpe, octathorp, octatherp

:Most scholars believe the word was invented by workers at the Bell Telephone Laboratories by 1968,{{cite magazine |url=http://www.svpal.org/~dickel/octothorp/Encore_magazine.pdf |magazine=Encore |title=Pressing Matters: Touch-tone phones spark debate |author=Hochhester, Sheldon |date=2006-09-29 |access-date=2006-12-17 |archive-date=2007-09-26 |archive-url=https://web.archive.org/web/20070926100216/http://www.svpal.org/~dickel/octothorp/Encore_magazine.pdf |url-status=live }} 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.Ralph Carlsen, "What the ####?" Telecoms Heritage Journal 28 (1996): 52–53. 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.{{cite web |url=http://dougkerr.net/pumpkin/articles/Octatherp.pdf |author=Douglas A. Kerr |title=The ASCII Character "Octatherp" |date=2006-05-07 |access-date=2010-08-23 |archive-date=2010-12-15 |archive-url=https://web.archive.org/web/20101215025150/http://dougkerr.net/pumpkin/articles/Octatherp.pdf |url-status=live }} 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 OglethorpeJohn Baugh, Robert Hass, Maxine H. Kingston, et al., "Octothorpe", The American Heritage Dictionary of the English Language (Boston: Houghton Mifflin, 2000) or using the Old English word for village, thorp, because the symbol looks like a village surrounded by eight fields.{{cite web|url=http://www.worldwidewords.org/weirdwords/ww-oct1.htm|title=Octothorpe|last=Quinion|first=Michael|date=19 May 2010|website=World Wide Words|access-date=10 May 2016|archive-date=18 May 2016|archive-url=https://web.archive.org/web/20160518172649/http://www.worldwidewords.org/weirdwords/ww-oct1.htm|url-status=live}}Bringhurst, "Octothorpe". Elements of Typographic Style The word was popularized within and outside Bell Labs."You Asked Us: About the * and # on the New Phones", The Calgary Herald, September 9, 1972, 90. 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".{{cite web|url=http://www.google.com/patents?vid=USPAT3920926|title=U.S. Patent No. 3,920,926|access-date=16 September 2014|archive-date=2 March 2013|archive-url=https://web.archive.org/web/20130302144808/http://www.google.com/patents?vid=USPAT3920926|url-status=dead}}

Sharp

: Use of the name "sharp" is due to the symbol's resemblance to {{unichar|266F|nlink=}}. 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'."{{cite web |url=https://learn.microsoft.com/en-us/dotnet/csharp/tour-of-csharp/ |title=A tour of the C# language |date=5 April 2023 |access-date=4 April 2024 |website=learn.microsoft.com |archive-date=8 March 2024 |archive-url=https://web.archive.org/web/20240308153937/https://learn.microsoft.com/en-us/dotnet/csharp/tour-of-csharp/ |url-status=live }} According to the ECMA-334 C# Language Specification, the name of the language is written "C#" ("{{resize|LATIN CAPITAL LETTER C}} (U+0043) followed by the {{resize|NUMBER SIGN}} # (U+0023)") and pronounced "C Sharp".{{cite book |url=https://ecma-international.org/wp-content/uploads/ECMA-334_7th_edition_december_2023.pdf |title=ECMA-334 C# language specification |edition=7th |publisher=Ecma International |page=xxiii |chapter=Introduction |date=December 2023 |access-date=4 April 2024 |archive-date=16 December 2023 |archive-url=https://web.archive.org/web/20231216004437/https://ecma-international.org/wp-content/uploads/ECMA-334_7th_edition_december_2023.pdf |url-status=live }}

Square

File:Detail-Tastatur-FeTAp-751-1982.JPG 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."{{cite web | url = http://www.itu.int/rec/T-REC-E.161-200102-I/en | title = E.161 : Arrangement of digits, letters and symbols on telephones and other devices that can be used for gaining access to a telephone network | publisher = International Telecommunication Union | date = 2 February 2001 | access-date = 23 December 2019 | archive-date = 2 November 2019 | archive-url = https://web.archive.org/web/20191102055736/https://www.itu.int/rec/T-REC-E.161-200102-I/en | url-status = live }} Formally, this is not a number sign but rather another character, {{unichar|2317|nlink=}}. The real or virtual keypads on almost all modern telephones use the simple {{code|#}} instead, as does most documentation.{{cn|date=January 2023}}

In North America, when {{angbr|#}} prefixes a number, it is read as "number". "A #2 pencil", for example, indicates "a number-two pencil". The abbreviations {{notatypo|'No.'}} and {{notatypo|'№'}} are used commonly and interchangeably. The use of {{angbr|#}} as an abbreviation for "number" is common in informal writing, but use in print is rare.{{cite web|url=

https://books.google.com/ngrams/graph?content=%22+%23+1+%22%2C%22+No.+1+%22&year_start=1800&year_end=2008&corpus=15&smoothing=3|title=

Google Ngram Viewer|access-date=

2021-02-05|archive-date=

2024-04-05|archive-url=

https://web.archive.org/web/20240405004325/https://books.google.com/ngrams/graph?content=%22+%23+1+%22%2C%22+No.+1+%22&year_start=1800&year_end=2008&corpus=15&smoothing=3|url-status=

live}} Where Americans might write "Symphony #5", British and Irish people usually write "Symphony No. 5".{{cn|date=November 2023}}

When {{angbr|#}} is after a number, it is read as "pound" or "pounds", meaning the unit of weight.{{cn|date=May 2025}} The text "5# bag of flour" would mean "five-pound bag of flour". This usage is very rare outside North America,

On telephone keypads, the {{keypress|#}} button is read in Northern America as "pound key" and in other Anglophone countries as "the hash key".

• In set theory, #S is one possible notation for the cardinality or size of the set S, instead of $|S|$. That is, for a set $S\; =\; \backslash \{s\_1,s\_2,s\_3,\; \backslash dots\; ,\; s\_n\backslash \}$, in which all $s\_i$ are mutually distinct, $\backslash \#S\; =\; n\; =\; |S|.$ This notation is only sometimes used for finite sets, usually in number theory, to avoid confusion with the divisibility symbol, e.g. $a\; \backslash mid\; b$.
• 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.

• In Unicode and ASCII, the symbol has a code point as {{unichar|0023}} and entity code {{code|#}} in HTML5.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 {{Webarchive|url=https://web.archive.org/web/20180401051616/http://www.w3.org/TR/html4/sgml/entities.html |date=2018-04-01 }} ("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 {{Webarchive|url=https://web.archive.org/web/20170805013240/http://www.w3.org/TR/2014/CR-html5-20140731/syntax.html#named-character-references |date=2017-08-05 }} ("num;").
• In many scripting languages and data file formats, especially ones that originated on Unix, {{code|#}} introduces a comment that goes to the end of the line.{{cite web |title=CSS Syntax and Selectors |url=https://www.w3schools.com/css/css_syntax.asp |website=W3Schools |access-date=2019-07-15 |archive-date=2019-07-12 |archive-url=https://web.archive.org/web/20190712051145/https://www.w3schools.com/css/css_syntax.asp |url-status=live }} The combination {{code|#!}} 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.
• {{code|#!}} is the symbol of the CrunchBang Linux distribution.
• In the Perl programming language, {{code|#}} 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 {{code|$array[$#array]}}. The number of elements in the array is {{code|$#array + 1}}, since Perl arrays default to using zero-based indices. If the array has not been defined, the return is also undefined. If the array is defined but has not had any elements assigned to it, e.g., {{code|1=@array = ()}}, then {{code|$#array}} returns {{code|−1}}. 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, {{code|#}} is used to start a preprocessor directive. Inside macros, after {{code|#define}}, it is used for various purposes; for example, the double pound (hash) sign {{code|##}} is used for token concatenation.
• In Unix shells, {{code|#}} is placed by convention at the end of a command prompt to denote that the user is working as root.
• {{code|#}} 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 {{code|https://en.wikipedia.org/wiki/Number_sign#Computing}} the portion after the {{code|#}} ({{code|Computing}}) is the fragment identifier, in this case denoting that the display should be moved to show the tag marked by {{code|1=...|2=html}} in the HTML.{{cite web|url=http://www.w3.org/TR/html4/intro/intro.html#fragment-uri|title=Introduction to HTML|access-date=16 September 2014|archive-date=16 August 2008|archive-url=https://web.archive.org/web/20080816014507/http://www.w3.org/TR/html4/intro/intro.html#fragment-uri|url-status=live}}
• Internet Relay Chat: on (IRC) servers, {{code|#}} precedes the name of every channel that is available across an entire IRC network.
• In blogs, {{code|#}} is sometimes used to denote a permalink for that particular weblog entry.
• In lightweight markup languages, such as wikitext, {{code|#}} is often used to introduce numbered list items.
• {{code|#}} 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 {{char|≠}}, being more intuitive than {{code|<>}} or {{code|1=!=}}. For example: {{nowrap|{{code|IF i # 0 THEN ...|cp}}}}
• In Rust, {{code|#}} is used for attributes such as in {{code|#[test]}}.
• In OCaml, {{code|#}} is the operator used to call a method.
• In Common Lisp,{{cite web|url=http://www.lispworks.com/documentation/HyperSpec/Body/02_dh.htm|title=Lispworks.com|access-date=16 September 2014|archive-date=10 October 2014|archive-url=https://web.archive.org/web/20141010014731/http://www.lispworks.com/documentation/HyperSpec/Body/02_dh.htm|url-status=live}} {{code|#}} 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, {{code|#}} is the prefix for certain syntax with special meaning.
• In Standard ML, {{code|#}}, when prefixed to a field name, becomes a projection function (function to access the field of a record or tuple); also, {{code|#}} prefixes a string literal to turn it into a character literal.
• In Mathematica syntax, {{code|#}}, when used as a variable, becomes a pure function (a placeholder that is mapped to any variable meeting the conditions).
• In LaTeX, {{code|#}}, when prefixing a number, references an arguments for a user defined command. For instance \newcommand{\code}[1]{\texttt{#1}}.
• In Javadoc,{{cite web|url=http://download.oracle.com/javase/1.5.0/docs/tooldocs/windows/javadoc.html#@see|title=Oracle.com|access-date=16 September 2014|archive-date=28 October 2011|archive-url=https://web.archive.org/web/20111028092204/http://download.oracle.com/javase/1.5.0/docs/tooldocs/windows/javadoc.html#@see|url-status=live}} {{code|#}} is used with the {{code|@see}} tag to introduce or separate a field, constructor, or method member from its containing class.
• In Redcode and some other dialects of assembly language, {{code|#}} is used to denote immediate mode addressing, e.g., {{code|LDA #10}}, which means "load accumulator A with the value 10" in MOS 6502 assembly language.
• in HTML, CSS, SVG, and other computing applications {{code|#}} is used to identify a color specified in hexadecimal format, e.g., {{code|#FFAA00}}. This usage comes from X11 color specifications, which inherited it from early assembler dialects that used {{code|#}} to prefix hexadecimal constants, e.g.: ZX Spectrum Z80 assembly.{{cite web|url=http://www.worldofspectrum.org/pub/sinclair/games-info/h/HiSoftDevpacV3.pdf|title=HISOFT DEVPAC ZX Spectrum Programmer's Manual|website=worldofspectrum.org|access-date=2017-10-03|archive-date=2018-11-12|archive-url=https://web.archive.org/web/20181112231113/http://www.worldofspectrum.org/pub/sinclair/games-info/h/HiSoftDevpacV3.pdf|url-status=live}}
• In Be-Music Script, every command line starts with {{code|#}}. 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".{{Cite magazine|url=https://time.com/2870942/hashtag-oed-oxford-english-dictionary/|title=You'll Never Guess the Real Name for a Hashtag|last=Nicks|first=Denver|date=June 13, 2014|magazine=TIME|access-date=May 5, 2016|archive-date=May 11, 2016|archive-url=https://web.archive.org/web/20160511235410/http://time.com/2870942/hashtag-oed-oxford-english-dictionary/|url-status=live}}
• In programming languages like PL/1 and Assembler used on IBM mainframe systems, as well as JCL (Job Control Language), the {{code|#}} (along with {{code|$}} and {{code|@}}) are used as additional letters in identifiers, labels and data set names.
• In J, {{code|#}} is the Tally or Count function,{{cite web|url=https://code.jsoftware.com/wiki/Vocabulary/number|title=Vocabulary/number|access-date=November 20, 2019|website=J NuVoc|archive-date=February 14, 2020|archive-url=https://web.archive.org/web/20200214105150/https://code.jsoftware.com/wiki/Vocabulary/number|url-status=live}} and similarly in Lua, {{code|#}} 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, {{code|#}} is a reference to the root namespace while {{code|##}} is a reference to the current space's parent namespace.
• In Ada, the {{code|#}} character is used in based integer literals, which take the form {{code|base#digits#}}, where {{code|base}} is an integer from 2 to 16 specifying the radix, and {{code|digits}} 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 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.{{cite web |last1=Vicars |first1=Bill |title=Lexicalization |url=http://www.lifeprint.com/asl101/topics/lexicalized_fingerspelling.htm |publisher=ASL University |access-date=6 September 2015 |archive-date=10 September 2015 |archive-url=https://web.archive.org/web/20150910183425/http://www.lifeprint.com/asl101/topics/lexicalized_fingerspelling.htm |url-status=live }}
• Copy writing and copy editing: Technical writers in press releases often use three number signs, {{char|###}} directly above the boilerplate or underneath the body copy, indicating to media that there is no further copy to come.{{cite web |title=###: What does ### mean at the end of a press release? |publisher=The Halo Group |url=https://www.thehalogroup.com/what-does-mean-at-the-end-of-a-press-release/ |first=Lara |last=Cohn |access-date=18 November 2021 |archive-date=17 November 2021 |archive-url=https://web.archive.org/web/20211117221859/https://www.thehalogroup.com/what-does-mean-at-the-end-of-a-press-release/ |url-status=live }}
• 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: {{char|#}} denotes a word boundary. For instance, {{code|/d/ → [t] / _#}} means that {{char|/d/}} becomes {{char|[t]}} 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.{{cite book|last=Carnie|first=Andrew|year=2006|title=Syntax: A Generative Introduction|location=Oxford|publisher=Wiley-Blackwell|isbn=1-4051-3384-8|edition=2nd|url-access=registration|url=https://archive.org/details/syntaxgenerative0000carn}}{{Cite book | first=R. L. | last=Trask | author-link=Larry Trask | title=A Dictionary of Grammatical Terms in Linguistics | location=London | publisher=Routledge | year=1993 | isbn=0-415-08627-2 | page=125}}
• Medical prescription drug delimiter: In some countries, such as Norway or Poland, {{char|#}} is used as a delimiter between different drugs on medical prescriptions.
• Medical shorthand: The hash is often used to indicate a bone fracture.{{Cite web |url=http://radiographics.rsnajnls.org/cgi/reprint/23/1/195.pdf |title=Glossary of Medical Devices and Procedures: Abbreviations, Acronyms, and Definitions |access-date=2008-05-16 |archive-date=2008-06-25 |archive-url=https://web.archive.org/web/20080625213552/http://radiographics.rsnajnls.org/cgi/reprint/23/1/195.pdf |url-status=live }} 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 {{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.{{cite web |url=http://www.merriam-webster.com/mw/table/proofrea.htm |title=Proofreaders' Marks |access-date=2020-09-03 |archive-date=2010-08-16 |archive-url=https://web.archive.org/web/20100816210104/http://www.merriam-webster.com/mw/table/proofrea.htm |url-status=dead }} from Merriam Webster
• 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.{{cite web|url=http://www.sfwa.org/wp-content/uploads/2009/06/Mssprep.pdf|title=Manuscript Preparation|last=McIntyre|first=Vonda|date=October 2008|website=sfwa.org|publisher=Science Fiction & Fantasy Writers of America|access-date=28 May 2020|archive-date=3 October 2020|archive-url=https://web.archive.org/web/20201003095936/http://www.sfwa.org/wp-content/uploads/2009/06/Mssprep.pdf|url-status=live}}
• 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.{{cite web|url=http://www.tucsonscrabble.com/articles/glossary.html|title=Scrabble Glossary|publisher=Tucson Scrabble Club|access-date=2012-02-06|archive-url=https://web.archive.org/web/20110830073348/http://www.tucsonscrabble.com/articles/glossary.html|archive-date=2011-08-30|url-status=dead}}
• 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. {{mono|# 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. In EBCDIC it is often at 0x7B or 0xEC.

Unicode characters with "number sign" in their names:

• {{unichar|0023}} (Other attested names in Unicode are: {{sc|pound sign (weight), hashtag, hash, crosshatch, octothorpe}}.{{cite web |author=Unicode Consortium |title=C0 Controls and Basic Latin| url=https://www.unicode.org/charts/PDF/U0000.pdf}})
• {{unichar|0600|html=|nlink=}}
• {{unichar|0BFA|html=|nlink=}}
• {{unichar|1AC6|nlink=}}
• {{unichar|FE5F}}
• {{unichar|FF03}}
• {{unichar|110BD|nlink=}}
• {{unichar|110CD|nlink=}}
• {{unichar|11FE9|nlink=}}
• {{unichar|E0023|nlink=}}

Additionally, a Unicode named sequence {{resize|KEYCAP NUMBER SIGN}} is defined for the grapheme cluster {{code|U+0023+FE0F+20E3}} (#️⃣).{{cite web |url=https://www.unicode.org/Public/UCD/latest/ucd/NamedSequences.txt |title=Unicode Named Character Sequences |work=Unicode Character Database |author=Unicode Consortium |author-link=Unicode Consortium |access-date=2020-07-16 |archive-date=2020-07-11 |archive-url=https://web.archive.org/web/20200711232858/http://www.unicode.org/Public/UCD/latest/ucd/NamedSequences.txt |url-status=live}}{{efn|{{unichar|0023}}, {{unichar|FE0F|nlink=}}, {{unichar|20E3|nlink=}} }}

On the standard US keyboard layout, the {{char|#}} symbol is {{keypress|Shift|3}}. On standard UK and some other European keyboards, the same keystrokes produce the Pound sign, and {{keypress|#}} may be moved to a separate key above the right shift key.

• looped square, {{char|⌘}}
• the Chinese character for "well" (wiktionary:井, {{langx|zh-Latn-pinyin|jǐng}})
• the game tic-tac-toe

{{For|uses of # within Wikipedia|Wikipedia:Number sign}}

{{Notelist}}

{{Reflist|2}}

{{navbox punctuation}}

Category:Typographical symbols