Graphical queries for the purpose of searching for pictorial information are of growing interest in areas where pictures provide valuable information, including, for instance, design, architecture, and engineering. Sketching graphical queries is a natural way of revealing the visual appearance of objects one has in mind. The problem which arises is identifying necessary shape properties of sketches, that is. those properties which are not accidental but are necessary for specifying a particular object property. This problem arises in particular because sketches are imprecise, and often distorted by the artistic limitations of the sketcher.