Number sign is a name for the symbol #, which is used for a variety of purposes, including (mainly in the United States) the designation of a number (for example, "#1" stands for "number one"). In recent years, it has been used for "hashtagging" on social media websites. The term number sign is most commonly used when the symbol is used before a number. In the United States, it is sometimes known as the pound sign, 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: # 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. 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 1900s, the Teletype Corporation was the first to use # to mean "number".[1] When the symbol follows a number, the symbol indicates weight in pounds. (Five pounds are indicated as 5#.)[2] This informal usage still finds handwritten use, and may be seen on some signs in markets and groceries. It is often claimed that the pound symbol 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 (HTML: ℔) 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 "//".[3] 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.[1] 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". In the United Kingdom and Ireland, the symbol is most often called the hash (a corruption of "hatch",[4] referring to cross-hatching). It is never used to denote pounds weight (lb is used for this non-metric unit) 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" may be understood in Britain and Ireland by some, it is generally not used. 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". 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.