# Number sign

The symbol # is most commonly known as the number sign,[1] hash,[2] or (in North American usage) pound sign.[3] 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 .[4]

#
Number sign
In UnicodeU+0023 # NUMBER SIGN (HTML `&#35;`)
Related
See alsoU+00A3 £ POUND SIGN (HTML `&#163;` · `&pound;`)
U+2116 NUMERO SIGN (HTML `&#8470;`)
Different from
Different fromU+266F MUSIC SHARP SIGN (HTML `&#9839;`)
U+2317 VIEWDATA SQUARE (HTML `&#8983;`)
U+22D5 EQUAL AND PARALLEL TO (HTML `&#8917;`)

Since 2007, widespread usage of the symbol to introduce metadata tags on social media platforms has led to such tags being known as "hashtags",[5] and from that, the symbol itself is sometimes called a hashtag.[6]

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

## History

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

The abbreviation written by Isaac Newton, showing the evolution from "℔" toward "#"

It is believed that the symbol traces its origins to the symbol "℔", an abbreviation of the Roman term libra pondo, which translates as "pound weight".[7][8] This abbreviation was printed with a dedicated ligature type, with a horizontal line across, so that the lowercase letter "l" would not be mistaken for the numeral "1". Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two slash-like strokes "//".[8] Examples of it being used to indicate pounds exist at least as far back as 1850.[9]

The symbol is described as the "number" character in an 1853 treatise on bookkeeping,[10] and its double meaning is described in a bookkeeping text from 1880.[11] The instruction manual of the Blickensderfer 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 the numero 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. Before this time, and still outside the United States, the term pound sign was used to refer to the pound currency symbol (£) or the pound weight symbol (lb).[16] An alternative theory is that the name pound sign arose from the fact that character encodings used the same code for both the number sign and the British Pound sign "£". Claims have included ISO 646-GB as well as the Baudot code in the late 19th century.[17] The apparent use of the sign to mean pounds weight in 1850 appears to rule out both of these code sets as the origin, although that same reference admits that the earliest reference in print was a decade after Baudot code.[9]

The term hash sign is found in South African writings from the late 1960s,[18] and from other non-North-American sources in the 1970s.[citation needed]

The symbol appears to be used primarily in handwritten material, while in the printing business, the numero symbol (№) and barred-lb (℔) are used for "number" and "pounds" respectively.

For mechanical devices, it appeared on the keyboard of the Remington Standard typewriter (c. 1886),[19] but was not used on the keyboards used for typesetting.[9] 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.[20]

### Usage in information technology

The number sign is often used in information technology to highlight a special meaning. It was adopted for use within internet relay chat (IRC) networks circa 1988 to label groups and topics.[21] This usage inspired[22] Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the microblogging network.[23] Although the hashtag started out most popularly on Twitter as the main social media platform for this use, the use has extended to other social media sites.[24]

## Usage in North America

Mainstream use in the United States is as follows: when it prefixes a number, it is read as "number", as in "a #2 pencil" (indicating "a number-two pencil"). The one exception is with the "#" key on a phone, which is always referred to as the pound key or pound. Thus instructions to dial an extension such as #77 are always read as "pound seven seven".[25]

In Canada the symbol is called both the number sign and the pound sign[26] or pound key. The American company Avaya has an option in their programming to denote Canadian English, which in turn instructs the system to say number sign to callers instead of pound sign.[27]

## Usage in the United Kingdom and Ireland

In the United Kingdom and Ireland, it is generally[28] called a hash (probably ultimately from hatch,[29] referring to cross-hatching, although the exact derivation is disputed). It is not used to denote pounds, either as weight (lb or lbs is used for this) or currency (pounds sterling, for which "£" is used). It is not called the pound sign; that term is understood to mean the currency symbol "£" for pound sterling or (formerly) Irish pound.

The use of "#" as an abbreviation for "number" may be understood in Britain and Ireland, where there has been business or educational contact with American usage, but use in print is rare[30] and British typewriters had "£" in place of the American "#".[31] Where Americans might write "Symphony #5", the British and Irish are more likely to write "Symphony No. 5".

To add to the confusion between "£" and "#", in BS 4730 (the UK national variant of the ISO/IEC 646 character set), 0x23 represents "£", whereas in ASCII (the US variant), it represents "#", thus it was common for the same character code to display "#" on US equipment and "£" on British equipment.

## Other names in English

The symbol has many other names (and uses) in English:

Comment sign
Taken from its use in many shell scripts and some programming languages (such as Python) to start comments.
Hash mark
Hashtag, or hashtag symbol
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.[6] Twitter documentation refers to it as "the hashtag symbol".[32]
Hex
Common usage 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.[33][34]
Octothorp, octothorpe, octathorp, octatherp
Most scholars believe the word was invented by workers at the Bell Telephone Laboratories by 1968,[35] 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.[36] 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.[37] The Merriam-Webster New Book of Word Histories, 1991, has a long article that is consistent with Doug Kerr's essay,[37] 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,[38] or using the Old English word for village, thorp, because the symbol looks like a village surrounded by eight fields.[39][40] The word was popularized within and outside Bell Labs.[41] 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".[42]
Sharp
Resemblance to the glyph used in music notation, U+266F (♯). So called in the name of the Microsoft programming languages C#, J# and F#. Microsoft says, "It's not the 'hash' (or pound) symbol as most people believe. It's actually supposed to be the musical sharp symbol. However, because the sharp symbol is not present on the standard keyboard, it's easier to type the hash symbol (#). The name of the language is, of course, pronounced 'see sharp'."[43] According to the ECMA-334 C# Language Specification, section 6, Acronyms and abbreviations, the name of the language is written "C#" ("LATIN CAPITAL LETTER C (U+0043) followed by the NUMBER SIGN # (U+0023)") and pronounced "C Sharp".[44]
Space
Used in proof-reading to denote that a space should be inserted. This can mean
1. a line space (the space between two adjacent lines denoted by line # in the margin),
2. a hair space (the space between two letters in a word, denoted by hr #)
3. a word space, or letter space (the space between two words on a line, two letter spaces being ##)
Em- and en-spaces (being the length of a letter m and n, respectively) are denoted by a square-shaped em- or en-quad character (⊞ and ⊟, respectively).[citation needed]
Square
Occasionally used in the UK (e.g. sometimes in BT publications and automatic messages) – especially during the Prestel era, when the symbol was a page address delimiter. The International Telecommunications Union specification ITU-T E.161 3.2.2 states: "The symbol is to be known as a 'square' or the most commonly used equivalent term in other languages".[45] Formally, this is not a number sign but rather another character, the Viewdata square.
Gate
From the 1960s to the 1990s the British telephone company, the GPO and its successors Post Office Telecommunications and British Telecom referred to this as gate on telephone keypads.
Others
tic tac toe, crosshatch, (garden) fence, mesh, flash, grid, pig-pen, tictactoe, scratch (mark), (garden) gate, hak, oof, rake, crunch, punch mark,[46] sink, corridor, capital 3, and waffle.[citation needed]

## In mathematics

• In set theory, #S is the cardinality of the set 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, ${\displaystyle \#S=n.}$
• 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 computing

• In Unicode and ASCII, the symbol has a code point as U+0023 # NUMBER SIGN (HTML `&#35;`) and `&num;` in HTML5.[47] It is graphically similar to several other symbols, including the sharp () from musical notation and the equal-and-parallel symbol (⋕) from mathematics and the Viewdata square from telephony, but is distinguished by its combination of level horizontal strokes and right-tilting vertical strokes.
• In many scripting languages and data file formats, especially ones that originated on Unix, the "#" introduces a comment that goes to the end of the line.[48] The combination `#!` at the start of an executable file is a "shebang", "hash-bang" or "pound-bang", used to tell the operating system which program to use to run the script (see magic number). This combination was chosen so it would be a comment in the scripting languages.
• `#!` is the symbol of the CrunchBang Linux distribution.
• In the Perl programming language, "#" 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 `\$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 on Array functions in the Perl language structure article.
• In both the C preprocessor and C++ preprocessor, as well as in other syntactically C-like languages, "#" is used to start a preprocessor directive. Inside macros (after #define) it is used for various purposes, including the double pound sign "##" used for token concatenation.
• In Unix shells, "#" is placed by convention at the end of a command prompt to denote that the user is working as root.
• "#" 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. For example, in the URL `http://en.wikipedia.org/wiki/Number_sign#In_computing` the portion after the "#" (`In_computing`) is the fragment identifier, in this case denoting that the display should be moved to show the tag marked by `<span id="In_computing">...</span>` in the HTML.[49]
• Internet Relay Chat: on (IRC) servers, "#" precedes the name of every channel that is available across an entire IRC network.
• In blogs, "#" is sometimes used to denote a permalink for that particular weblog entry.
• In lightweight markup languages, such as wikitext, "#" is often used to introduce numbered list items.
• "#" 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 ≠, being more intuitive than <> or !=. For example: `IF i # 0 THEN ...`
• In OCaml, "#" is the operator used to call a method.
• In Common Lisp,[50] "#" 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, "#" is the prefix for certain syntax with special meaning.
• In Standard ML, "#", when prefixed to a field name, becomes a projection function (function to access the field of a record or tuple); also, "#" prefixes a string literal to turn it into a character literal.
• In Mathematica syntax, "#", when used as a variable, becomes a pure function (a placeholder that is mapped to any variable meeting the conditions).
• In LaTeX, "#", when prefixing a number, references an arguments for a user defined command. For instance `\newcommand{\code}[1]{\texttt{#1}}`.
• In Javadoc,[51] "#" is used with the @see tag to introduce or separate a field, constructor, or method member from its containing class.
• In some dialects of assembly language, "#" is used to denote immediate mode addressing, e.g., LDA #10, which means "load the accumulator with the value 10" in MOS 6502 assembly language.
• in HTML, CSS, SVG, and other computing applications "#" is used to identify a color specified in hexadecimal format, e.g., `#FFAA00`. This usage comes from X11 color specifications, which inherited it from early assembler dialects that used "#" to prefix hexadecimal constants, e.g.: ZX Spectrum Z80 assembly.[52]
• In Be-Music Script, every command line starts with "#". Lines starting with characters other than "#" are treated as comments.
• The use of the hash symbol in a hashtag 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", but this is technically incorrect.[53]
• In programming languages like PL/1 and Assembler used on IBM mainframe systems, as well as JCL (Job Control Language), the "#" (along with "\$" and "@") are used as additional letters in identifiers, labels and data set names.
• In J, "#" is the Tally or Count function.[54], and similarly in Lua, "#" can be used as a shortcut to get the length of a table, or get the length of a string. Due to the ease of writing "#" over longer function names, this practice has become standard in the Lua community.
• In Dyalog APL, "#" is a reference to the root namespace while "##" is a reference to the current space's parent namespace.

## Other uses

• Press releases: The notation "###" denotes "end", i.e. that there is no further copy to come.[55]
• Chess notation: A hash after a move denotes checkmate, being easier to type than the traditional "‡".
• 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.[56]
• Prescription drug delimiter: In some countries, such as Norway or Poland, "#" is used as a delimiter between different drugs on medical prescriptions.
• Copy writing and editing: Technical writers often use three hash signs ("###") as a marker in text where more content will be added or there are errors to be corrected.
• Mining: In underground mining, the hash sign is sometimes used as a shorthand for "seam" or "shaft". An example would be "4#", which would mean "four shaft" or "four seam" depending on the context.[citation needed]
• Medical shorthand: The hash is often used to indicate a bone fracture.[57] 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 "#" to denote either the number of treatments in a prescription (e.g. 60Gy in 30#), or the fraction number (#9 of 25).
• Linguistic phonology: "#" 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).
• 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.[58]
• Teletext and DVB subtitles (in the UK and Ireland): The hash symbol 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 #
• 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.[59]
• 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).

## Unicode

In Unicode, several # characters are assigned:

• U+0023 # NUMBER SIGN (HTML `&#35;`). Other attested names in Unicode are: pound sign, hash, crosshatch, octothorpe.
• U+FF03 FULLWIDTH NUMBER SIGN (HTML `&#65283;`)
• U+FE5F SMALL NUMBER SIGN (HTML `&#65119;`)

At least three orthographically distinct number signs from other languages are also assigned:

## On keyboards

On the standard US keyboard layout, the # symbol is ⇧ Shift+3. On standard UK and some other European keyboards, the same keystrokes produce the £ symbol, and # may be moved to a separate key above the right shift key. If there is no key the symbol can be produced on Windows with Alt+35, on Mac OS with ⌥ Opt+3, and on Linux with Compose++.

## References

1. ^ "number sign". Oxford English Dictionary.
2. ^ "hash". Oxford English Dictionary.
3. ^ "pound sign". Oxford English Dictionary. Retrieved 5 May 2016.
4. ^ Houston, Keith (20 October 2014). Shady Characters: The Secret Life of Punctuation, Symbols, and Other Typographical Marks. W W Norton & Company.
5. ^ Piercy, Joseph (25 October 2013). Symbols: A Universal Language. Michael OMara. pp. 84–85. ISBN 978-1-78243-073-5. Retrieved 4 October 2014.
6. ^ a b "Why is the symbol # called the hashtag in Twitter?". Merriam-Webster's Learner's Dictionary.
7. ^ "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." Keith Gordon Irwin, in The Romance of Writing, p. 125. The Unicode character U+2114 L B BAR SYMBOL (HTML `&#8468;`) is intended to represent this ligature.
8. ^ a b Houston, Keith (2013-09-06). "The Ancient Roots of Punctuation". The New Yorker. Retrieved 16 October 2013.
9. ^ a b c "The Sign of the Number". Sentence Spacing. 2015-10-06. Retrieved 24 November 2015.
10. ^ Crittendon, S. W. (1853). An Elementary Treatise on Book-keeping by Single and Double Entry. Philadelphia: E., C., & J. Biddle. p. 10. Retrieved 24 November 2015.
11. ^ Duff, C. P.; Duff, W. H.; Duff, R. P. (1880). Book-Keeping By Single and Double Entry. Harper and Brothers. p. 21. Retrieved 24 November 2015.
12. ^ n.a. (1896). Method of Operating and Instructions for Practice on the Blickensderfer Typewriter (PDF). Atlanta, GA: K. M. Turner. p. 14. It is best to use the 'number mark' for plus; the hyphen for minus, and two hyphens for the sign =
13. ^ e.g. J. W. Marley, "The Detection and Illustration of Forgery By Comparison of Handwriting", in Proceedings of the Sixteenth Annual Convention of the Kansas Bankers' Association. Kansas City: Rusell. 1903. p. 180.
14. ^ e.g. The British Printer vol. viii (1895), p. 395
15. ^ Thurston, Ernest L. (1917). Business Arithmetic for Secondary Schools. New York: Macmillan. p. 419. business symbols pound.
16. ^ Lawrence, Nancy M.; F. Ethel McAfee; Mildred M. Butler (1932). Correlated studies in stenography. Gregg. p. 141.
17. ^ "The 'pound sign' mystery". Retrieved 22 December 2012.
18. ^ Research Review. Navorsingsoorsig vols. 18–21, pp. 117, 259 (1968)
19. ^ "Remington Standard typewriter". New York: Wyckoff, Seamans & Benedict. 1886. p. 50.
20. ^ Keith Houston (2013). "The Octothorpe". Shady Characters: The Secret Life of Punctuation, Symbols, and Other Typographical Marks. W. W. Norton & Company. pp. 41–57.
21. ^ "Channel Scope". Section 2.2. RFC 2811
22. ^ "#OriginStory". Carnegie Mellon University. August 29, 2014.
23. ^ Parker, Ashley (June 10, 2011). "Twitter's Secret Handshake". The New York Times. Retrieved July 26, 2011.
24. ^ Mashable, By Christina Warren. "Facebook finally gets #hashtags - CNN.com". CNN. Retrieved July 16, 2006.
25. ^ William Safire (March 24, 1991). "On Language; Hit the Pound Sign". The New York Times. Retrieved May 21, 2011.
26. ^ Barber, Katherine, ed. (2004). The Canadian Oxford dictionary (2nd ed.). Toronto: Oxford University Press. ISBN 0195418166.
28. ^ "How the # became the sign of our times". The Guardian. Retrieved 30 December 2014.
29. ^ "Hash sign". Oxford English Dictionary. Retrieved 14 October 2013.
30. ^
31. ^ "The Hashtag: A History Deeper than Twitter". Retrieved 30 December 2014.
33. ^ Jack Tsen-Ta Lee. "A Dictionary of Singlish and Singapore English". Retrieved 14 January 2016.
34. ^ "Address Formats". Retrieved 14 January 2016.
35. ^ Hochhester, Sheldon (2006-09-29). "Pressing Matters: Touch-tone phones spark debate" (PDF). Encore.
36. ^ Ralph Carlsen, "What the ####?" Telecoms Heritage Journal 28 (1996): 52–53.
37. ^ a b Douglas A. Kerr (2006-05-07). "The ASCII Character "Octatherp"" (PDF). Cite journal requires `|journal=` (help)
38. ^ John Baugh, Robert Hass, Maxine H. Kingston, et al., "Octothorpe," The American Heritage Dictionary of the English Language (Boston: Houghton Mifflin, 2000)
39. ^ Quinion, Michael (19 May 2010). "Octothorpe". World Wide Words. Retrieved 10 May 2016.
40. ^ Bringhurst, "Octothorpe". Elements of Typographic Style
41. ^ "You Asked Us: About the * and # on the New Phones," The Calgary Herald, September 9, 1972, 90.
42. ^ "U.S. Patent No. 3,920,926". Retrieved 16 September 2014.
44. ^ "Ecma-international.com". Retrieved 16 September 2014.
45. ^ "E.161 : Arrangement of digits, letters and symbols on telephones and other devices that can be used for gaining access to a telephone network". International Telecommunications Union. 2 February 2001. Retrieved 23 December 2019.
46. ^ "Pronunciation guide for Unix - Bash - SS64.com". Retrieved 16 September 2014.
47. ^ 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 ("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 ("num;").
48. ^ "CSS Syntax and Selectors". W3Schools. Retrieved 2019-07-15.
49. ^ "Introduction to HTML". Retrieved 16 September 2014.
50. ^ "Lispworks.com". Retrieved 16 September 2014.
51. ^ "Oracle.com". Retrieved 16 September 2014.
52. ^ "HISOFT DEVPAC ZX Spectrum Programmer's Manual" (PDF). worldofspectrum.org.
53. ^ Nicks, Denver (June 13, 2014). "You'll Never Guess the Real Name for a Hashtag". TIME. Retrieved May 5, 2016.
54. ^ "Vocabulary/number". J NuVoc. Retrieved November 20, 2019.
55. ^
56. ^ "Scrabble Glossary". Tucson Scrabble Club. Retrieved 2012-02-06.
57. ^ Glossary of Medical Devices and Procedures: Abbreviations, Acronyms, and Definitions
58. ^ Carnie, Andrew (2006). Syntax: A Generative Introduction (2nd ed.). Oxford: Wiley-Blackwell. ISBN 1-4051-3384-8.
59. ^ Vicars, Bill. "Lexicalization". ASL University. Retrieved 6 September 2015.