Number sign

Not to be confused with the Chinese character 井, the sharp sign (♯), the viewdata square (⌗), the numero sign (№), the equal and parallel to symbol (⋕), or the game Tic-tac-toe's grid.

For uses of number signs within Wikipedia, see Wikipedia:Number sign.

Number sign is a name for the symbol #, which is used for a variety of purposes, including (mainly in Canada and the United States) the designation of a number (for example, "#1" stands for "number one"). On many social media platforms, it is used to declare a searchable metadata tag called a hashtag.

The term number sign is most commonly used when the symbol is used before a number. In the United States and Canada, it is sometimes known as the pound sign (particularly in the context of its use on telephone keypads), and has been traditionally used in the food industry as an abbreviation for pounds avoirdupois. Outside of North America the symbol is called hash and the corresponding telephone key is called the "hash key", and the term "pound sign" usually describes the British currency symbol "£". The symbol is defined in Unicode as U+0023 # NUMBER SIGN (HTML #<dot-separator> as in ASCII).

The symbol may be confused with the musical symbol called sharp (♯). In both symbols, there are two pairs of parallel lines. The main difference is that the number sign has two horizontal strokes while the sharp sign has two slanted parallel lines which must rise from left to right, in order to avoid being obscured by the horizontal musical staff lines. Although the signs are strictly speaking different, one of the names for the telephone number sign key in French is dièse (sharp sign), and the name of the programming language C sharp is usually rendered with a hash symbol rather than the technically correct sharp sign.

Origin and names

It is often claimed that the use of the number sign for pound derives from a series of abbreviations for pound, the unit of weight.

According to this suggestion, the symbol goes back to the abbreviation lb. for "pound" (Roman term libra pondo or "pound weight"); 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 forward-slash-like strokes "//".

Alternative theories suggest that the name "pound sign" is a result of the fact that character encodings have historically 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, however its usage as a pound sign predates all of these code sets. Examples of this usage exist at least as far back as 1850, and its double meaning is described in a book-keeping text from 1880.

The symbol is described as the "number" character in an 1853 treatise on book-keeping. 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. It appeared on the keyboard of the Remington Standard typewriter (c. 1886), but was not used on the keyboards used for typesetting.

The instruction manual of the Blickensderfer model 5 appears to refer to the symbol as the "number mark". Some early twentieth-century U.S. sources refer to it as the "number sign", although this could also refer to the numero sign.

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 U.S., the term "pound sign" was used to refer to the pound currency symbol (£) or the pound weight symbol (lb).

"Hash sign" is found in South Africans writings from the late 1960s, and from other non-North-American sources in the 1970s.

Usage in North America

Mainstream use in the United States is as follows: when it precedes a number, it is read as "number", as in "a #2 pencil" (spoken aloud as: "a number-two pencil"). When the symbol follows a number, the symbol indicates weight in pounds. (Five pounds are indicated as 5#.) This traditional usage still finds handwritten use, and may be seen on some signs in markets and groceries.

In Canada the symbol is called both the "number sign" and the "pound sign". Avaya 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 sign".

Usage in the United Kingdom and Ireland

In the United Kingdom and Ireland, it is generally called the hash (probably ultimately from "hatch", referring to cross-hatching, although the exact derivation disputed). It is never used to denote pounds weight (lb or lbs is used for this) or pounds sterling (for which "£" is used). It is never called the "pound sign", because 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 by some, especially where there has been business or educational contact with American usage, but use in print is rare and British typewriters had "£" in place of the American "#". Where Americans might write "Symphony #5", the British and Irish are more likely to write "Symphony No. 5", or perhaps use the numero sign "Symphony № 5" (as in The New Grove Dictionary of Music and Musicians).

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 "#". It was thus common, when systems were incorrectly configured, for "£" to be displayed as "#" and vice versa.

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.

Cross 

In China, non-native English speakers often refer to the number sign as "cross". It is said as jĭng in Chinese, as it looks like the Chinese character for water well ("井").

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'.

Octothorp, octothorpe, octathorp, octatherp

Used by Bell Labs engineers by 1968. Lauren Asplund says that he and a colleague were the source of octothorp at AT&T engineering in New York in 1964. The Merriam-Webster New Book of Word Histories, 1991, has a long article that is consistent with Doug Kerr's essay, in that it says "octotherp" was the original spelling, and that the word arose in the 1960s among telephone engineers as a joke. 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 

Resemblance to the glyph used in music notation, U+266F (♯). So called in the name of the Microsoft programming languages C#, J# and F#. However 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".

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).

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 # is to be known as a 'square' or the most commonly used equivalent term in other languages."

Others 

crosshatch, (garden) fence, mesh, flash, grid, pig-pen, tictactoe, scratch (mark), (garden) gate, hak, oof, rake, crunch, punch mark, sink, corridor, capital 3, and waffle.

In mathematics

• In set theory, #S is the cardinality of the set S. That is, for a set $S = \{s_1,s_2,s_3, \dots , s_n\}$, $\#S = n.$
• In topology, where A and B are manifolds, A#B is the manifolds' connected sum. In knot theory (a branch of topology), where A and B are knots, A#B is the knots' knot sum.
• In number theory, n# is the primorial of n.

In computing

• In ARM assembly language it is used to denote an immediate value. This is one that can be evaluated during program compilation (usually a constant value).
• 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. The combination #! at the start of an executable file is a "shebang" or "hash-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., @array's last element is at $array[$#array]. The number of elements in @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 the C preprocessor (and the C++ preprocessor, and 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 webpage or other resource to introduce a "fragment identifier" – an id which defines a position within that resource. For example, in the URL https://infogalactic.com/info/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.
• 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.
• On social networking sites such as Twitter, # is used to denote a metadata tag, or hashtag. This influence has also spread into television, such as the hashtag heel wrestler in WWE.
• In lightweight markup languages, such as wikitext, # is often used to introduce numbered list items.
• In OCaml, # is the operator used to call a method.
• In Common Lisp, # 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, # 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 some obscure languages that used "#" to prefix hexadecimal constants.
• In Be-Music Script, every command line starts with #. Lines starting with characters other than # are treated as comments.

Other uses

• The use of the # 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.
• Press releases: the notation "###" denotes "end", i.e. that there is no further copy to come.
• Chess notation: # 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.
• 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.
• Medical shorthand: # is often used to indicate a bone fracture. For example, '#NOF' is often used for 'fractured neck of femur'.
• In 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).
• In linguistic syntax, # before an example sentence denotes that the sentence is semantically ill-formed, though grammatically well-formed. For instance, "#The toothbrush is pregnant" is a grammatical sentence, but the meaning is odd.
• In Teletext and DVB subtitles in the UK, the # 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 into English uses the # prefixing an all-caps word to identify 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.

Unicode

In Unicode, several # characters are assigned:

• U+0023 # NUMBER SIGN (HTML #) Other attested names in Unicode are: pound sign, hash, crosshatch, octothorpe.

• U+FF03 ＃ FULLWIDTH NUMBER SIGN (HTML ＃)

• U+FE5F ﹟ SMALL NUMBER SIGN (HTML ﹟)

• U+E0023 TAG NUMBER SIGN (HTML 󠀣)

In other languages or scripts:

• U+0600 ؀ ARABIC NUMBER SIGN (HTML ؀)

• U+0BFA ௺ TAMIL NUMBER SIGN (HTML ௺)

• U+110BD KAITHI NUMBER SIGN (HTML 𑂽)

Related characters, the sharp sign in musical notation:

• U+266F ♯ MUSIC SHARP SIGN (HTML ♯)

• U+1D12A 𝄪 MUSICAL SYMBOL DOUBLE SHARP (HTML 𝄪)

• U+1D130 𝄰 MUSICAL SYMBOL SHARP UP (HTML 𝄰)

• U+1D131 𝄱 MUSICAL SYMBOL SHARP DOWN (HTML 𝄱)

• U+1D132 𝄲 MUSICAL SYMBOL QUARTER TONE SHARP (HTML 𝄲)

In mathematics, the equal and parallel to sign:

• U+22D5 ⋕ EQUAL AND PARALLEL TO (HTML ⋕)

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 pound currency symbol (£), and # is moved to a separate key above the right shift. On UK Mac keyboards, # is generated by ⌥ Opt+3, whereas on other European Mac keyboards, the # can be found above the right shift key.

See also

• Numero sign (№)