Peppr can operate openly since prostitution, and the advertising of prostitution, are both legal in Germany. But even where they are not, the internet is transforming the sex trade. Prostitutes and punters have always struggled to find each other, and to find out what they want to know before pairing off. Phone-box "tart cards" for blonde bombshells and leggy se?oritas could only catch so many eyes.

Customers knew little about the nature and quality of the services on offer. Personal recommendations, though helpful, were awkward to come by. Sex workers did not know what risks they were taking on with clients.

Now specialist websites and apps are allowing information to flow between buyer and seller, making it easier to strike mutually satisfactory deals. The sex trade is becoming easier to enter and safer to work in: prostitutes can warn each other about violent clients, and do background and health checks before taking a booking. Personal web pages allow them to advertise and arrange meetings online; their clients' feedback on review sites helps others to proceed with confidence.