This chapter is devoted to a fairly detailed description, in strictly
geometrical terms always, of the seventeen planar crystallographic groups
(wallpaper patterns). No attempt to prove that there exist precisely
seventeen types is made in this chapter, but the Crystallographic
Restriction is proven in full detail in section 4.0 (which may just be
browsed through at first reading). Emphasis is placed on how wallpaper
patterns may be derived from each other or from border patterns.