trackball

A trackball is a pointing device consisting of a ball held by a socket containing sensors to detect a rotation of the ball about two axes—like an upside-down mouse with an exposed protruding ball. Users roll the ball to position the on-screen pointer, using their thumb, fingers, or commonly the palm of the hand while using the fingertips to press the mouse buttons.
With most trackballs, operators have to lift their finger, thumb or hand and reposition in on the ball to continue rolling, whereas a mouse would have to be lifted itself and re-positioned. Some trackballs have notably low friction, as well as being made of a dense material such as glass, so they can be spun to make them coast. The trackball's buttons may be situated to that of a mouse or to a unique style that suits the user.
Large trackballs are common on CAD workstations for easy precision. Before the advent of the touchpad, small trackballs were common on portable computers (such as the BlackBerry Tour) where there may be no desk space on which to run a mouse. Some small "thumbballs" are designed to clip onto the side of the keyboard and have integral buttons with the same function as mouse buttons.

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