Twitter co-founder defends app and talks about new project
21 November 2014 Last updated at 18:18 GMT
The chairman and co-founder of Twitter, Jack Dorsey, has been in London for the global launch of his new company 'Square'.
The free app turns a tablet or smartphone into a sales register that can be used to process payments.
He spoke to the BBC's Rory Cellan-Jones and defended Twitter against criticism including its use by terrorists and its power to end politicians' careers.