A thread containing moving images is being used as a security feature by the Bank of England for the first time on its new £50 note, which has come into circulation.
The green "motion thread" has five windows featuring the pound symbol and the number 50, which move up and down when the note is tilted from side to side.
The thread has been woven into the new-style note, which celebrates the 18th century business partnership of entrepreneur Matthew Boulton and engineer James Watt, rather than printed on to it.
When the note is tilted up and down, the images move from side to side and the number 50 and the pound symbol switch, the Bank of England said.
The Boulton and Watt note is the first Bank of England note to feature a motion thread, as well as marking the first time two portraits have appeared together on the back of a Bank of England bank note.
It will also be the first banknote in circulation to be signed by Chris Salmon, who was appointed the Bank's executive director, banking services and chief cashier in April.
He said: "The Boulton and Watt £50 banknote has new and enhanced security features which demonstrate the Bank's commitment to its role of maintaining public confidence in the currency.
"The motion thread security feature is one of the new measures which should help members of the public to identify genuine £50 banknotes."
Boulton was a leading entrepreneur of the Industrial Revolution.
In 1775, he went into partnership with Watt, whose improvements to steam engine design had led to greater efficiency and cheaper running costs.