On this SAGE terminal, the operator uses a light gun and operates the console controls to select targets on the screen to gather more information, choose the appropriate defence, and give the order to attack local sites. Communication takes place in real time between the user and the information given by the computer. This demonstrates that an interactive interface with graphical representations (not just words or text) can be designed, implemented and used effectively. SAGE (Semi-Automatic Ground Environment) is the national air defence system put in place by the United States after 1949 and the explosion of the first Soviet atomic bomb on 29 August 1949. This real-time air defence system was designed at the Massachusetts Institute of Technology (MIT), which in turn recruited companies and other organisations to design what would be a web-based system covering all of North America. The heart of this digital system – the AN/FSQ-7 computer – was developed, built and maintained by IBM. SAGE was one of the largest computer projects in the world in the 1950s. Between 1952 and 1955, it generated 80 per cent of IBM’s revenue from computers and, by 1958, over 7,000 IBM employees were involved in the project. SAGE gave rise to many technological innovations that IBM incorporated into other computer products.