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The number sign ("#"), also called the “pound sign” or “hash sign,” is a specific digital and print character that is represented by the ASCII value 35 or the binary input 010-0011. Although it is not an alphanumeric character, it has been present in earlier technologies, such as in typewriter keyboards and telephone keypads, and still gets a lot of use in cutting-edge technologies, such as social media.
Reports on the history of this sign, which is also called the octothorpe, show that it was created at Bell Laboratories to add to the telephone a way of sending instructions to the phone operator.
Since then, this once obscure character had actually been used for a lot of things. In modern telecommunications, it is frequently used in digital answering services, virtual telephone assistance and other setups as a way to indicate the end of an entry or to indicate a menu option. Phone users who have used automated answering services are familiar with the digital voice that instructs the caller to “press the pound key,” which refers to the number sign on the telephone keypad.
In the age of social media, the number sign or pound sign is well known for its use in hashtags. In fact, this character was actually known previously as the hash sign, which predated many of its other names. In a hashtag, the number sign precedes a word or phrase or collection of characters that represents a searchable index or a metadata that describes the writer's feelings in a kind of modern shorthand, e.g., "#excited".