This was a digitiser in which the tip of the pen contained a small spark gap. Several times a second, a spark would be triggered which made a little ``tick.'' The digitiser measured the time the sound arrived at two microphones and, knowing the speed of sound, calculated the position of the pen. AutoCAD supported two models, the 18x24 inch GP-7 and the 60x72 inch GP-8. A 3D model was also available. The sparks created ozone, which contributed to the electric feeling of pioneering desktop CAD. The pen was also handy for chasing away pesky cats.