<hardware> A pointing device consisting of a ball housed in
a socket containing sensors to detect rotation of the ball
about two axes - like an upside-down mouse. The user rolls
the ball with his thumb or the palm of his hand to move a
cursor. Tracker balls are common on CAD workstations for
ease of use and on modern portable computers, where there
may be no desk space on which to use a mouse. Some clip
onto the side of the keyboard and have integral buttons which
have the same function as mouse buttons.
