A virtual world is simply a computer-generated scene which simulates a physical world to a greater or lesser degree, depending upon whether one includes elaborate paraphernalia for enhancing the sense of reality, simply a computer screen, or some middle level of equipment. In any case, the computer creates a sense of verisimilitude which can be truly awe-inspiring, and we can all imagine the wonder which can be brought into a museum if the visitor can participate in such a process and truly see the setting from which objects came - whether that be an individual building or a complex urban landscape. To give one simple example, imagine what a difference it would make if the British Museum could take visitors to Athens and let them see the Elgin marbles as if they were standing on the Acropolis in the Fifth Century B.C. rather than in a museum in modern London. At the root of a virtual world (or a walk-through) is an image or group of images of the world being portrayed. The root image may be a manufactured one, made up completely of whole cloth, or it may be a real one, carefully made to represent reality. If the latter, the root image must finally be based upon dimensions and spatial positions so that it can conform to reality. This is where computer-assisted drafting and design (CADD) programs come in, because these are the programs which can create the root images for virtual reality systems - root images based on the real world rather than an imaginary one.