As an object moves relative to a viewpoint, its appearance changes. In this paper we analyze the topological constraints on the changing appearance of line drawings of objects as the objects or the camera move. We start with a Huffman-Clowes junction dictionary. We show a way of deriving vertex types from junction types by inference rather than by table look-up. We then derive three constraints on the change in appearance of an object: conservation of vertices, conservation of vertex type, and conservation of adjacencies. Using these constraints, we develop a matching algorithm that traces vertices from one image to the next. Examples are given showing correct matching tor simple objects, including partially visible objects and multiple objects in the same scene.