Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online.To work with a system, users have to be able to control the system and assess the state of the system. For example, when driving an automobile, the driver uses the steering wheel to control the direction of the vehicle, and the accelerator pedal, brake pedal and gearstick to control the speed of the vehicle. The driver perceives the position of the vehicle by looking through the windscreen and exact speed of the vehicle by reading the speedometer. The user interface of the automobile is on the whole composed of the instruments the driver can use to accomplish the tasks of driving and maintaining the automobile.