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A trackball is a controller device similar to a mouse, where a ball is used to control onscreen movement. However, where the mouse requires the controller to be moved around in order for the ball inside it to move, trackballs simply require the ball to be moved since the exposed end with the ball is facing up instead of down.
Trackballs were first featured in Atari arcade games such as Football, Basketball, Missile Command, and Centipede, and then eventually became optional controllers for home game consoles and personal computers. Some trackballs are specifically designed to work as alternates for mouse controllers.