A task completed using a computer will generally involve at least three models: one for the data that describes the task; one for the user actions that can be carried out on that data; and one that establishes the cultural context in which it is carried out. The data model determines what is physically possible through the selection of parts and their relationships, the same way that di erent vehicles (car, bicycle or truck) emerge from choices of frame, axles, tires, power train, and power source. The data model determines, to a great extent, the a ordances of the resulting vehicle (how much it can carry, how you steer it), and those a ordances establish an interface between the mechanism and the user. Finally, the user’s goals and the vehicle’s a ordances interact with the cultural context-the rules of the road-to further establish whether and how the task can be accomplished.