As mentioned before, drawing is done by filled, two-dimensional polygons. There are three kinds of user-manipulable polygons which all can have an arbitrary number of vertices. A new polygon is created by selecting the desired type from a menu (Figure 8), then selecting the feature or control point to attach it to (unless it is a free polygon). A polygon is moved by dragging it; its vertices are changed by dragging them to the desired locations. A new polygon always has eight vertices, which can be deleted or added to as desired.