The # symbol you see under every Instagram post was once just a Roman weight measurement. It took centuries to become social media's hashtag

You probably see the # symbol several times a day while scrolling Instagram, whether it is attached to a celebrity post, a trending topic or a random phrase people are using to get more attention. Today, most people simply call it a hashtag. But the symbol itself is much older than social media, and its story goes back to the Romans and their way of writing a measurement for weight.

Before it became one of the most recognisable symbols on the internet, # had several other jobs and names. Its journey took it from handwritten abbreviations to typewriters, telephone keypads and eventually social media.

How a Roman pound became the # symbol

According to language expert and content creator RobWords, the story starts with the Roman word “libra”, which was used for a pound of weight. The abbreviation “lb” eventually became associated with the pound measurement that is still used today.


When people wrote “lb” by hand, they would sometimes draw a line across the letters to show that they were writing an abbreviation. Over time, the two letters and the line were written more quickly and in different styles.

One version became particularly important. A messy handwritten form of “lb”, including the crossing line, is believed to have gradually developed into the symbol we now recognise as #. RobWords has pointed to the handwriting of Sir Isaac Newton as an example of how such an abbreviation could become a much more simplified mark.

Eventually, the complicated handwritten form was reduced to two horizontal and two vertical strokes. The modern hash or number sign was taking shape.

The symbol had a life before hashtags

The # symbol did not suddenly appear when social media arrived. It had already found its way onto typewriters by the late 19th century, where it was commonly used to mean “number”, as in #2.

It later entered the world of telephones too. Bell Laboratories included the symbol on touch-tone keypads in 1968, alongside the asterisk. The keys could be used for commands in telephone-based computer systems.

The symbol also goes by several names. In Britain, “hash” is common, while Americans have traditionally called it the “pound sign”. “Number sign” is another familiar name.

There is also the rather unusual technical name “octothorpe”. The term was coined by engineers at Bell Laboratories in the 1960s, although it never became the everyday name most people use.

When # became a hashtag

The social media meaning came much later. Encyclopedia Britannica notes that technology developer Chris Messina suggested using the # symbol on Twitter in 2007 to group posts around common subjects.

On August 23, 2007, Messina wrote: “[H]ow do you feel about using # (pound) for groups. As in #barcamp [msg]?”

Blogger Stowe Boyd used the term “hashtag” shortly afterwards. The idea gained more attention when hashtags were used to organise posts about the 2007 San Diego wildfire, allowing people to follow updates more easily.

Twitter later made hashtags clickable, and other platforms followed. Instagram, Facebook, YouTube, LinkedIn and several others eventually adopted the system.

Today, a hashtag is essentially a word or phrase preceded by # that helps organise related posts and makes them easier to find. So the next time you tap a hashtag on Instagram, remember that the symbol began its journey as something much less digital: a shorthand connected to the Roman pound.