IS MONEY a good medium to spread messages? At first Alexei Navalny, a Russian opposition activist and noted blogger, was sceptical. But then he did the maths: if 5,000 Russians stamped 100 bills each, every citizen would encounter at least one of the altered notes as they passed from person to person.

Members of Iran’s Green Movement used this tactic in 2009, writing slogans on banknotes during their anti-government protests. This prompted a ruling that defaced notes would no longer be accepted by banks. Similarly, supporters of the Occupy movement have added slogans and infographics about income inequality to dollar bills. And members of China’s Falun Gong movement wrote messages on banknotes attacking government persecution.

The use of money as a communications medium, distributing words and images as it passes from hand to hand, is ancient. The earliest coins, minted in Lydia (now part of Turkey) in the 7th century BC, depicted the head of a lion, thought to have been a royal symbol. Later rulers had their names and images inscribed on coins, along with symbolic images of various kinds. In the era before printing, this was a very efficient way to project their image directly to the people.

But their subjects were also aware of the messaging power of money, as the recently revamped exhibit on the history of money at the British Museum in London reveals. It includes a Roman coin from 215AD, on which the Christian “chi-rho” symbol has been scratched behind the emperor’s head; a French coin from 1855 overstamped with an advertisement for Pears Soap; and a 1903 British penny on which Edward VII’s face has been stamped with “Votes for women” by suffragettes (all pictured). Mr Navalny’s call for Russians to stamp messages on banknotes is just the latest incarnation of a centuries-old idea—a pioneering example of what we now call social media.