This chapter describes the concepts of descriptive geometry as applied to solving spatial problems. The basic geometric elements of points, lines, and planes, used extensively in traditional descriptive geometry applications, are defined by example problems. These 2-D geometric elements can be combined with 3-D geometric primitives to solve design problems, utilizing 3-D CAD and solid modeling software. Figure 14.1 illustrates the application of descriptive geometry concepts in the design of a chemical plant. For the plant to function safely, pipes must be placed to intersect correctly and to clear each other by a specified distance, and they must correctly intersect the walls of buildings. Descriptive geometry is used to solve these types of spatial problems.