Nearly every Android device supports the useful Acceleration Sensor, but another sensor also gives you physical information. The Orientation Sensor tells how the phone is oriented relative to the Earth. This sensor provides roll, pitch, and azimuth data.
Imagine for a moment that the phone is an airplane (Figure 10.13). Looking at the phone as you normally would, the top (earpiece) would be the nose of the plane and the bottom (the home button and so on) would be the tail end. The roll is how much the plane rotates without the tail or nose moving, or how much the wings move up and down. The pitch is how much higher or lower the nose is than the tail.