The concept of structure contours, introduced in chapter 5, is advanced in this chapter. To succesfull interpretate geological maps and visualize subsurface attitude of geological surfaces, the ability to think clearly in three dimensions is a prerequisite. This chapters calls for that ability specifically. In the first section, it is explained how structure-contour maps for complex surfaces are obtained. Form-line contour maps, a related way of visualization, is introduced in the second paragraph. Then the theory of structure contours is applied to horizontal folds and plunging folds in the following two section. The chapter concludes with the effect of topography on the map patterns of folded sequences.