15. An apparatus comprising: means for tracking a location of a pen instrument when the pen instrument is used to modify a physical document, wherein sensors within the pen instrument track the location of the pen instrument; means for generating signals describing movement of the pen instrument based on the location of the pen instrument; means for transmitting the signals to a receiving device, wherein the signals are used to modify an electronic document corresponding to the physical document, and further wherein strokes made with the pen instrument are displayed when the electronic document is displayed.
15. An apparatus comprising: means for tracking a location of a pen instrument when the pen instrument is used to modify a physical document, wherein sensors within the pen instrument track the location of the pen instrument; means for generating signals describing movement of the pen instrument based on the location of the pen instrument; means for transmitting the signals to a receiving device, wherein the signals are used to modify an electronic document corresponding to the physical document, and further wherein strokes made with the pen instrument are displayed when the electronic document is displayed. 21. The apparatus of claim 15 wherein the means for tracking a location of a pen instrument comprises means for receiving an infrared signal from at least two infrared radiating scanning sources located at prescribed positions external to the pen instrument, the scanning sources to illuminate the pen instrument, wherein the pen instrument determines movements based, at least in part, on the infrared signals.