A Graphic Input Device comprises a pen-shaped pointing device moving about a writing surface and using ultrasonic (US) waves propagating in the air to measure displacement. Can be used in either a 2D or 3D mode. Uses displacement measuring means for two directions in space, each parallel to the writing surface, and comprising means to establish an US waves path between an US transmitter and an US receiver; means to measure changes in the phase of the received signal, and for computing the displacement. Means for measuring displacement in a third dimension are optionally provided.
