Traditional computer drafting systems and three- dimensional geometric modelling systems work in bottom-up fashion. They provide a range of graphic primitives, such as vectors, arcs, and splines, together with operators for inserting, deleting, combining, and transforming instances of these. Thus they are conceptually very similar to word processors, with the difference that they operate on two- dimensional or three-dimensional patterns of graphic primitives rather than one-dimensional strings of characters. This sort of system is effective for input and editing of drawings or models that represent existing designs, but provides little more help than a pencil when you want to construct from scratch a drawing of some complex object such as a human figure, an automobile, or a classical column: you must depend on your own knowledge of what the pieces are and how to shape them and put them together. If you already know how to draw something then a computer drafting system will help you to do so efficiently, but if you do not know how to begin, or how to develop and refine the drawing, then the efficiency that you gain is of little practical consequence. And accelerated performance, flashier color graphics, or futuristic three-dimensional modes of interaction will not help with this problem at all. By contrast, experienced expert graphic artists and designers usually work in top-down fashion-beginning with a very schematic sketch of the whole object, then refining this, in step-by-step fashion, till the requisite level of precision and completeness is reached. For example, a figure drawing might begin as a “stick figurei schema showing lengths and angles of limbs, then be developed to show the general blocking of masses, and finally be resolved down to the finest details of contour and surface. Similarly, an architectural drawing might begin as a parti showing just a skeleton of construction lines, then be developed into a single-line floor plan, then a plan showing accurate wall thicknesses and openings, and finally a fully developed and detailed drawing.