Number sign

Number sign is a name for the symbol #, which is used for a variety of purposes including, in some countries, the designation of a number (for example, "#1" stands for "number one"). The symbol is in Unicode as code point U+0023 # number sign; it is also present in ASCII with the same value.

In Commonwealth English, the symbol is usually called the hash and the corresponding telephone key is called the hash key. In American English, the symbol is usually called the pound sign (outside the US, this term often refers instead to the British currency symbol "£") and the telephone key is called the pound key.^[1] In Canadian English, this key is most frequently called the pound key but also in some circumstances the number sign key. Beginning in the 1960s, telephone engineers have attempted to coin a special name for this symbol, with variant spellings including octothorp, octothorpe, octathorp, octotherp, octathorpe, and octatherp;^[2] none of these has become widely accepted.

In many parts of the world, including Australia, Canadian French, Russia, and parts of Europe, number sign (or equivalents in local languages) refers instead to the numero sign ("№").

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. Thus, only the number sign may have an italic appearance.

Usage and naming conventions in North America

Mainstream use in the US as follows: when it precedes a number, it is read as "number", as in "a #2 pencil" (spoken aloud as: "a number two pencil").

In the United States, the symbol is traditionally called the pound sign. The pound name derives from a series of abbreviations for pound, the unit of weight. 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/digit "1". Unicode character U+2114 (℔) is called the "L B bar symbol", and it 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 "//".^[1] 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. The business clerks' hurried way of writing the abbreviation appears to have been responsible for the # sign used for pound."

In Canada, the symbol is traditionally referred to as the number sign. Major telephone equipment manufacturers, such as Nortel, have an option in their programming to denote Canadian pronunciation, which in turn instructs the system to say "number sign" to callers instead of "pound sign." This same option causes the system to say "zed" instead of the United States' "zee" for the letter Z.

Usage in the UK and Ireland

In the UK and Ireland, the symbol is most often called the hash. It is never used to refer to pounds weight (lb is commonly used for this) or to pounds sterling (where "£" is used). It is never called the "pound sign" since that term is understood to mean the currency symbol "£", for pound sterling or (formerly) Irish pound.

The use of "#" as an abbreviation for "number" is uncommon in Britain, to the extent that many people would not even recognise the usage. Where Americans might write "Symphony #5", the British 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 "#". It was thus common, when systems were incorrectly configured, for "£" to be displayed as "#" or 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 Perl) to start comments.

Cross 

In China, non-native English speakers often refer to the number sign as "cross". It is said as jing 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 (sic: number sign) key'. The term 'hex' is discouraged in Singapore in favour of 'hash'.

Octothorp, octothorpe, octathorp, octatherp

Used by Bell Labs engineers by 1968.^[3] Lauren Asplund, who provided the article, 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,^[4] 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".^[5]

Sharp 

Resemblance to the glyph used in music notation, U+266F (♯). So called in the name of the Microsoft programming languages C# 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'."^[6] 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".^[7]

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."

Crunch 

It is used as "crunch" in the Linux distribution #! "Crunch Bang" (http://crunchbanglinux.org/)

Others 

crosshatch, (garden) fence, mesh, flash, grid, pig-pen, tictactoe, scratch (mark), (garden) gate, hak, oof, rake, sink, corridor, unequal, punch mark.^[8]

In mathematics

• In set theory, #S is the cardinality of the set S. That is, for a set #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

• # is one of the two standard special keys beyond digits 0 to 9 on a telephone keypad (the other being the star key, *). It generates a compound tone mixing 941 Hz and 1477 Hz. Its function depends on services provided by a given telephone-based service, but it is often used to denote the end of a variable-length number such as an account number or item number.
• 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.
• 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 <a name="In_computing">...</a> in the HTML ^[9]
• 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".
• In lightweight markup languages, such as wikitext, # is often used to introduce numbered list items.
• In Objective Caml, # is the operator used to call a method.
• In Common Lisp^[10] 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^[11], # is used with the @see tag to introduce or separate a field, constructor, or method member from its containing class.

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 ‡.
• 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".^[12]
• 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 (ie. 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.^[13]
• 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. eg. # For he's a jolly good fellow #
• On Twitter and some other websites based on user-generated content, terms in posts are prefixed with a # to categorize them under that term (e.g. #iranelection); this feature is known as hashtagging.

On keyboards

On standard US keyboard layouts, the # symbol is <kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">⇧ Shift</kbd>+<kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">3</kbd>. 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 <kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">⌥ Opt</kbd>+<kbd class="keyboard-key" style="border: 1px solid; border-color: #ddd #bbb #bbb #ddd; border-bottom-width: 2px; -moz-border-radius: 3px; -webkit-border-radius: 3px; border-radius: 3px; background-color: #f9f9f9; padding: 1px 3px; font-family: inherit; font-size: 0.85em; white-space: nowrap;">3</kbd>, whereas on European Mac keyboards, the # can be found above the right shift key. Under DOS and Microsoft Windows, it can be also generated through the Alt code Alt-35.

References

1. ^ ^{a b} William Safire. "On Language; Hit the Pound Sign". New York Times. http://www.nytimes.com/1991/03/24/magazine/on-language-hit-the-pound-sign.html. Retrieved 2011-05-21. 
2. ^ Octothorpe on Dictonary.com (which has its own sources cited)
3. ^ Hochhester, Sheldon (2006-09-29). "Pressing Matters: Touch-tone phones spark debate". Encore. http://www.svpal.org/~dickel/octothorp/Encore_magazine.pdf. 
4. ^ Douglas A. Kerr (2006-05-07) (PDF). The ASCII Character "Octatherp". http://dougkerr.net/pumpkin/articles/Octatherp.pdf. 
5. ^ U.S. Patent No. 3,920,296, Google Patent Search
6. ^ Frequently Asked Questions about C#
7. ^ Ecma-international.com
8. ^ http://ss64.com/bash/syntax-pronounce.html
9. ^ "Introduction to HTML", W3C Recommendation
10. ^ Lispworks.com
11. ^ Oracle.com
12. ^ Glossary of Medical Devices and Procedures: Abbreviations, Acronyms, and Definitions
13. ^ Carnie, Andrew (2006). Syntax: A Generative Introduction (2nd ed.). Oxford: Wiley-Blackwell. ISBN 1405133848.