hatch

The # symbol has a complicated genealogy. One origin traces to the Latin abbreviation 'lb' for libra pondo ('pound weight'), which scribes wrote with a horizontal bar through the top of the 'l'—a stroke that, rendered hastily, evolved into the # shape. Another tradition connects it to the abbreviation 'No.' for number, where a careless pen stroke produced the double-crossed shape. Typographers called it the 'octothorpe,' the 'pound sign,' the 'number sign,' the 'hash mark,' or simply 'hash'—the name 'hash' possibly derived from 'hatch,' the criss-crossed lines of hatching in engraving and drawing.

Bell Laboratories engineers added the # key to the telephone keypad in 1968 when designing the touch-tone phone, pairing it with the * key to give operators additional signaling options. For decades, automated phone systems used # as the 'pound key'—'press pound to return to the main menu.' The symbol was functional but inert: it performed operations rather than carrying meaning.

On August 23, 2007, programmer and internet activist Chris Messina posted to Twitter: 'how do you feel about using # (pound) for groups. As in #barcamp?' He was proposing that the # symbol be used to tag topics, creating searchable, linked categories. Twitter initially ignored the suggestion. But the 2007 San Diego wildfires changed everything: users spontaneously tagged posts with #sandiegofire, creating a real-time information stream the platform couldn't ignore. Twitter made hashtags clickable in 2009.

Today, hashtag is a word—a noun, a verb, a cultural practice. Instagram, TikTok, LinkedIn, and dozens of other platforms use it. The word 'hashtag' itself was added to the Oxford English Dictionary in 2014. The # symbol accumulated its meanings over centuries—weight, number, space, telephone command—before Twitter gave it the one meaning that eclipsed all the others. The criss-crossed lines of medieval scribes now organize global conversation.