Number sign

Number sign is a name for the symbol #, which is used for a variety of purposes, including the designation of a number (for example, "#1" stands for "number one"). The symbol is defined in Unicode as U+0023 # NUMBER SIGN (# · as in ASCII).

The term Number sign is popular only in Canada. In most of the United States the symbol is usually called the pound sign and the telephone key is called the "pound key". Outside of North America the symbol is called hash and the corresponding telephone key is called the "hash key" (and the term "pound sign" often describes the British currency symbol "£")

The symbol is easily confused with the musical symbol called sharp (♯). In both symbols, there are two pairs of parallel lines. The key difference is that the number sign has true horizontal strokes while the sharp sign has two slanted parallel lines which must rise from left to right, in order to avoid being confused with the musical staff lines. Both signs may have true vertical lines; however, they are compulsory in the sharp sign, but optional in the number sign (#) depending on typeface or handwriting style.

Origin and usage and naming conventions 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"). A theory claims that back in early 1900, the Teletype Corporation was the first to use # to mean "number".

It is often claimed that the pound name derives from a series of abbreviations for pound, the unit of weight. The theory goes that at first "lb." was used; however, printers later designed a font containing a special symbol of an "lb" with a line through the verticals so that the lowercase letter "l" would not be mistaken for the numeral "1". Unicode character U+2114 ℔ L B BAR SYMBOL is a cursive development of this symbol. Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two forward-slash-like strokes "//". Keith Gordon Irwin, in The Romance of Writing, p. 125, says "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.

An alternative theory is that the name “pound sign” is a result of the fact that character encodings have historically used the same code for both the hash symbol and the British pound sign "£". It is sometimes supposed that the problem originated in ISO 646-GB, but it seems more likely that it has its origin in Baudot code in the late 19th century.

In Canada the symbol is commonly called the number sign. Major telephone-equipment manufacturers, such as Nortel, 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, the symbol is most often called the hash (probably ultimately a corruption of "hatch"). It is never used to denote pounds weight (lb is commonly used for this) or pounds sterling (where "£" 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.

While the use of "#" as an abbreviation for "number" is widely understood in Britain and Ireland, it is generally only used for technical reasons, for instance lists, itineraries, manuals etc. and not so much in formal writing. Where Americans might write "Symphony #5", the British and Irish are more likely to write "Symphony No. 5", or use the numero sign "Symphony № 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 "#". 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 which 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 by editors to denote where space should be inserted in a galley proof. 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, sink, corridor, crunch, punch mark.

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 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 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.
• 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 and 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.

Other uses

• 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 as medical shorthand for "fracture".
• In linguistic phonetics, # 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 sung either by a character or heard in background music. e.g. # For he's a jolly good fellow #
• The use of the # symbols as '[[1]]' - a phenomenon conceived by Chris Messina, and popularised by social media network twitter, as a way to direct conversations and topics amongst users.

Unicode

In Unicode, several # characters are assigned:

• U+0023 # NUMBER SIGN (# · Other accepted names in Unicode are: pound sign, hash, crosshatch, octothorpe)
• U+FE5F ﹟ SMALL NUMBER SIGN
• U+FF03 ＃ FULLWIDTH NUMBER SIGN
• U+E0023 TAG NUMBER SIGN

In other languages or scripts:

• U+0600 ؀ ARABIC NUMBER SIGN
• U+0BFA ௺ TAMIL NUMBER SIGN
• U+110BD KAITHI NUMBER SIGN

Related characters, the sharp sign in musical notation:

• U+266F ♯ MUSIC SHARP SIGN (♯)
• U+1D12A 𝄪 MUSICAL SYMBOL DOUBLE SHARP
• U+1D130 𝄰 MUSICAL SYMBOL SHARP UP
• U+1D131 𝄱 MUSICAL SYMBOL SHARP DOWN
• U+1D132 𝄲 MUSICAL SYMBOL QUARTER TONE SHARP

On keyboards

On the standard US keyboard layout, the # symbol is ⇧ Shift+3. On standard UK and some 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 European Mac keyboards, the # can be found above the right shift key.