In one mode of an interface, a bounded area is displayed on a mobile device. A straight line is displayed that divides the bounded area into first and second areas. An angle of the line indicates an orientation of a first axis of the mobile device relative to a respective vector. In response to detecting the change in orientation of the second axis of the device relative to the vector, while maintaining the angle of the line at a first angle, a relative size of the first and second areas is changed in accordance with the change in orientation of the second axis. In a second mode, first and second bounded shapes are displayed. Positions of the bounded shapes with respect to each other are changed along one or more axes in response to a change in orientation of a surface of the mobile device relative to a respective vector.