Editing Number sign (section)

== Usage ==
When {{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>{{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}}</ref> or outside North America. Instead the abbreviation {{notatypo|'No.'}} or the symbol {{notatypo|'№'}} or just "number" is used.{{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". Again this is rare outside North America.

=== Mathematics ===
* In [[set theory]], #''S'' is one possible notation for the [[cardinality]] or size of the [[set (mathematics)|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 set]]s, 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 [[manifold]]s ''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|#P]] denotes a [[complexity class]] of [[counting problem (complexity)|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]] and [[ASCII]], the symbol has a [[code point]] as {{unichar|0023}} and [[entity code]] {{code|&num;}} 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 {{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;").</ref>
* 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.<ref name="hash character">{{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 }}</ref> The combination {{code|#!}} at the start of an executable file is a "[[Shebang (Unix)|shebang]]", "hash-bang" or "pound-bang", used to tell the operating system which program to use to run the script (see [[Magic number (programming)|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 data type|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 [[Perl language structure#Array functions|Array functions]] in the Perl language structure article.
* In both the [[C preprocessor|C]] and [[C++]] preprocessors, as well as in other syntactically C-like languages, {{code|#}} is used to start a preprocessor [[Directive (programming)|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 shell]]s, {{code|#}} is placed by convention at the end of a [[command prompt]] to denote that the user is working as [[Superuser|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 [[HTML element#Anchor|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=<span id="Computing">...</span>|2=html}} in the HTML.<ref>{{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}}</ref>
* [[Internet Relay Chat]]: on (IRC) servers, {{code|#}} precedes the name of every [[IRC channels|channel]] that is available across an entire IRC network.
* In [[blog]]s, {{code|#}} is sometimes used to denote a [[permalink]] for that particular weblog entry.
* In [[lightweight markup language]]s, such as [[wiki]]text, {{code|#}} is often used to introduce numbered list items.
* {{code|#}} is used in the [[Modula-2]] and [[Oberon (programming language)|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 (programming language)|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]],<ref>{{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}}</ref> {{code|#}} is a dispatching [[Lisp reader|read macro]] character used to extend the [[S-expression]] syntax with short cuts and support for various data types ([[complex number]]s, vectors and more).
* In [[Scheme (programming language)|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 [[Wolfram Mathematica|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 <syntaxhighlight lang="tex" inline>\newcommand{\code}[1]{\texttt{#1}}</syntaxhighlight>.
* In [[Javadoc]],<ref>{{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}}</ref> {{code|#}} is used with the {{code|@see}} tag to introduce or separate a field, constructor, or method member from its containing class.
* In [[Core War|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]], [[Cascading Style Sheets|CSS]], [[Scalable Vector Graphics|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 [[Zilog Z80|Z80]] assembly.<ref>{{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}}</ref>
* 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 (open-source advocate)|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">{{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}}</ref>
* 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 (programming language)|J]], {{code|#}} is the ''Tally'' or ''Count'' function,<ref>{{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}}</ref> and similarly in [[Lua (programming language)|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 (programming language)|APL]], {{code|#}} is a reference to the root [[namespace]] while {{code|##}} is a reference to the current space's parent namespace.
* In [[Ada (programming language)|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).

=== Other uses ===
* [[Algebraic notation (chess)|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>{{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 }}</ref>
* [[copy writer|Copy writing]] and [[copy editing]]: Technical writers in [[press release]]s 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.<ref>{{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 }}</ref>
* [[Note (typography)|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 (mark)|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.<ref>{{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}}</ref><ref>{{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}}</ref>
* [[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]].<ref>{{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 }}</ref> For example, "#NOF" is often used for "fractured [[Femur neck|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 [[List of proofreader's marks|proofreading mark]], to indicate that a space should be inserted.<ref>{{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]]</ref>
* Publishing: When submitting a science fiction manuscript for publication, a number sign on a line by itself (indented or centered)  indicates a [[Section (typography)|section break]] in the text.<ref>{{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}}</ref>
* [[Scrabble]]: Putting a number sign after a word indicates that the word is found in the British word lists, but not the [[Official Tournament and Club Word List|North American lists.]]<ref>{{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}}</ref>
* [[Teletext]] and [[Digital Video Broadcasting|DVB]] [[subtitle]]s (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|''#&nbsp;For he's a jolly good fellow&nbsp;#''}}