Curves form the backbone of geometric modeling and creating solid models. A sketch consists of multiple curves that are connected to form closed contours (loops). Loops may be nested. A sketch that consists of only one contour generates a solid without holes, whereas a sketch with multiple contours generates a solid with holes in it. Figure 8.1 shows examples. A one-contour sketch is demonstrated in Figure 8.1A. You can create a sketch with only one-level nesting; that is, an outside contour with one or more disconnected contours inside it, as shown in Figure 8.1B. However, if you attempt to use more than one-level nesting as shown in Figure 8.1C, the solid creation operation fails.