Number sign

Engineers at Bell Labs gave this sign a new name, octothorpe, when they put it on push-button telephones in the 1960s. It is also the number sign, the hash and, in the United States, the pound sign. A popular explanation links its shape to “lb” with a line through it, the abbreviation for the Roman pound, libra.

Hashtags began in 2007, when Chris Messina suggested marking Twitter posts with # to gather conversations by subject. The musical sharp ♯ looks similar yet is a separate character: on a sharp the crossing strokes slope, while on # it is usually the upright strokes that lean.