Abstract: A computer system comprises a keyboard (11), a display screen (13), a first pair of position-sensing electrodes (18.1, 18.2), a second pair of position-sensing electrodes (20.1, 20.2), a signal injection electrode (22), and an oscillator (27). The oscillator injects a signal via the signal injection electrode and the operator's left hand (L) into the operator's body, and this creates a field around the operator's right hand (R). The position-sensing electrodes are arranged underneath the keyboard and sense the strength of the field. This enables the position of the operator's right hand (R) in an X-Y plane above the keyboard to be determined. To this end the position-sensitive electrodes are connected via synchronous detectors (34, 44) and an analogue-to-digital converter (36) to a microprocessor (46), which operates to control the position of a cursor on the display screen.