On 21 June 1948, the 'Baby' becomes the first computer to execute a program stored in addressable read-write electronic memory (Williams-Kilburn tube memory, a cathode-ray tube used as storage). This validates the random-access memory concept later used widely, and leads in 1949 to the Manchester Mark I, which introduces index registers. In February 1951, Ferranti Ltd's commercial derivative becomes the first electronic computer marketed as a standard product delivered to a customer.