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The symbol is known variously in English-speaking regions as the number sign, hash, or pound sign. 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 .

(2024). 9780393064421, W. W. Norton & Company.

Since 2007, widespread usage of the symbol to introduce on platforms has led to such tags being known as "",

(2013). 9781782430735, Michael OMara.
and from that, the symbol itself is sometimes called a hashtag.

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


History
It is believed that the symbol traces its origins to the symbol , an abbreviation of the Roman term , which translates as "pound weight". The abbreviation "lb" was printed as a dedicated ligature including a horizontal line across (which indicated abbreviation). Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two slash-like strokes "//".

The symbol is described as the "number" character in an 1853 treatise on , and its double meaning is described in a bookkeeping text from 1880. The instruction manual of the Blickensderfer model 5 typewriter () appears to refer to the symbol as the "number mark". 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 although this could also refer to the ().e.g. 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)". The use of the phrase "pound sign" to refer to this symbol is found from 1932 in U.S. usage. The term hash sign is found in South African writings from the late 1960s Research Review. Navorsingsoorsig vols. 18–21, pp. 117, 259 (1968) and from other non-North-American sources in the 1970s.

For mechanical devices, the symbol appeared on the keyboard of the Remington Standard typewriter (). It appeared in many of the early teleprinter codes and from there was copied to , 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 (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. This usage inspired Chris Messina to propose a similar system to be used on to tag topics of interest on the microblogging network; this became known as a . Although used initially and most popularly on Twitter, hashtag use has extended to other social media sites.


Names
Number sign
"Number sign" is the name chosen by the Unicode consortium. Most common in Canada
(2024). 9780195418163, Oxford University Press.
and the northeastern United States. American telephone equipment companies which serve Canadian callers often have an option in their programming to denote , which in turn instructs the system to say number sign to callers instead of pound.
Pound sign or pound
"Pound sign" or "pound" are the most common names used in the United States, where the "#" key on a phone is commonly referred to as the pound key or simply pound. Dialing instructions to an extension such as #77, for example, can be read as "pound seven seven". This name is rarely used outside the United States, where the term pound sign is understood to mean .
Hash, ,
In the United Kingdom, Australia, and some other countries, it is generally called a "hash" (probably from "hatch", referring to cross-).
Programmers also use this term; for instance is "hash, bang" or "shebang".
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".
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.
, octothorpe, octathorp, octatherp
Most scholars believe the word was invented by workers at the by 1968, 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 , 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. 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 John 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, , because the symbol looks like a village surrounded by eight fields.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".
Sharp
Use of the name "sharp" is due to the symbol's resemblance to . The same derivation is seen in the name of the programming languages C#, 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'." 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".
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." Formally, this is not a number sign but rather another character, . The real or virtual keypads on almost all modern telephones use the simple instead, as does most documentation.
Other
Names that may be seen include: , crunch, fence, flash, garden fence, garden gate, gate, grid, hak, mesh, oof, pig-pen, punch mark, rake, scratch, scratch mark, , and unequal.


Usage
When prefixes a number, it is read as "number". "A #2 pencil", for example, indicates "a number-two pencil". The abbreviations 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. Where Americans might write "Symphony #5", British and Irish people usually write "Symphony No. 5".

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.

is ''not'' a replacement for the [[pound sign]] , but British typewriters and keyboards have a  key where American keyboards have a  key. Many early computer and teleprinter codes (such as BS 4730 (the UK national variant of the ISO/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).
     


Mathematics
  • In , # S is one possible notation for the or size of the set S, instead of |S|. That is, for a set S = \{s_1,s_2,s_3, \dots , s_n\}, in which all s_i are mutually distinct, \#S = n = |S|. This notation is only sometimes used for , usually in , to avoid confusion with the symbol, e.g. a \mid b.
  • In , A# B is the of A and B, or of knots A and B in .
  • In , n# is the of n.
  • In constructive mathematics, # denotes an apartness relation.
  • In computational complexity theory, #P denotes a of counting problems. The standard notation for this class uses the number sign symbol, not the 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 and , the symbol has a ("The following sections present the complete lists of character entity references." Https://www.w3.org/TR/2014/CR-html5-20140731/syntax.html#named-character-references ("num;").
  • 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. 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 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 . The number of elements in the array is , 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., , then returns . See the section on Array functions in the Perl language structure article.
  • In both the and C++ preprocessors, as well as in other syntactically C-like languages, is used to start a preprocessor directive. Inside macros, after , it is used for various purposes; for example, the double pound (hash) sign is used for token .
  • In , is placed by convention at the end of a to denote that the user is working as .
  • is used in a of a 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 the portion after the () is the fragment identifier, in this case denoting that the display should be moved to show the tag marked by in the HTML.
  • Internet Relay Chat: on (IRC) servers, precedes the name of every that is available across an entire IRC network.
  • In , is sometimes used to denote a for that particular weblog entry.
  • In lightweight markup languages, such as , is often used to introduce numbered list items.
  • is used in the Modula-2 and Oberon programming languages designed by and in the 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:
  • In Rust, is used for attributes such as in .
  • In , is the operator used to call a method.
  • In , is a dispatching character used to extend the syntax with short cuts and support for various data types (, vectors and more).
  • In Scheme, is the prefix for certain syntax with special meaning.
  • In , , when prefixed to a field name, becomes a projection function (function to access the field of a record or tuple); also, prefixes a 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 , , when prefixing a number, references an arguments for a user defined command. For instance \newcommand{\code}1{\texttt{#1}}.
  • In , is used with the tag to introduce or separate a field, constructor, or method member from its containing class.
  • In and some other dialects of assembly language, is used to denote immediate mode addressing, e.g., , which means "load accumulator A with the value 10" in MOS 6502 assembly language.
  • in , CSS, SVG, and other computing applications is used to identify a color specified in format, e.g., . 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.
  • In , every command line starts with . Lines starting with characters other than "#" are treated as comments.
  • The use of the hash symbol in a is a phenomenon conceived by Chris Messina, and popularized by social media network , 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".
  • 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, 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 while is a reference to the current space's parent namespace.


Other uses
  • Algebraic notation for chess: A hash after a move denotes .
  • 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.
  • and : Technical writers in 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.
  • Footnote symbols (or endnote symbols): Due to ready availability in many fonts and directly on computer keyboards, "#" and other symbols (such as the ) have in recent years begun to be occasionally used in catalogues and reports in place of more traditional symbols (esp. dagger, double-dagger, ).
  • Linguistic : denotes a word boundary. For instance, means that becomes when it is the last segment in a word (i.e. when it appears before a word boundary).
  • Linguistic : 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.
    (2024). 9781405133845, Wiley-Blackwell. .
    (1993). 9780415086271, Routledge.
  • Medical prescription drug delimiter: In some countries, such as or , is used as a between different drugs on medical prescriptions.
  • Medical shorthand: The hash is often used to indicate a . For example, "#NOF" is often used for "fractured ". 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).
  • : The notation denotes "end", i.e. that there is no further copy to come.
  • As a proofreading mark, to indicate that a space should be inserted. from
  • 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.
  • : Putting a number sign after a word indicates that the word is found in the British word lists, but not the North American lists.
  • and DVB (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.


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

Unicode characters with "number sign" in their names:

  • Other attested names in Unicode are: .

Additionally, a Unicode named sequence is defined for the (#️⃣).


On keyboards
On the standard , the symbol is . On standard UK and some other European keyboards, the same keystrokes produce the , 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 , on Mac OS with , and on Linux with .


See also


Explanatory notes
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