Coordinate systems and reference frames exist for the pleasure and convenience of the engineer who defines them. In the next chapters we will freely adorn our systems with multiple coordinate systems as we see fit, to aid in understanding and solving the prob-lem. We will denote one of these as the global or absolute coordinate system, and the others will be local coordinate systems within the global framework. The global sys-tem is often taken to be attached to Mother Earth, though it could as well be attached to another ground plane such as the frame of an automobile. If our goal is to analyze the motion of a windshield wiper blade, we may not care to include the gross motion of the automobile in the analysis.