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"). 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 (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 # · as in ASCII).
The symbol may be confused with the musical symbol called sharp (♯). In both symbols, there are
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 "//". 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.
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