A touch pad (also called track pad) is a pointing device consisting of specialized surface that can translate the motion and position of a user’s fingers to a relative position on screen. They are a common feature of laptop computers and also used as a substitute for a computer mouse where desk space is limited. Touch pads vary in size but are rarely made larger than 40 square centimeters (6.3 X 6.3 cm or about 6 inch2). They can also be found on personal digital assistants (PDAs) and some portable media players, such as the ipod using the click wheel. A touchpad is perhaps the most common kind of tactile sensor.