Design is an important but little understood intelligent activity. Conceptual design is the transformation of functional specifications to an initial concept of an artifact that achieves them. Human designers rely heavily on the use of sketches, which can be thought of as qualitative models of a device. An appealing model of conceptual design is that of a mapping from qualitative functional specifications to a corresponding qualitative object model. As a case study, I have investigated this model for the conceptual design of part shapes in elementary mechanisms, such as ratchets or gears. I present a qualitative modelling forrealism for mechanical function, the place vocabulary, and for shape, the metric diagram. I show that qualitative functional attributes can not be mapped into corresponding qualitative attributes of a device that achieves them, and consequently that qualitative function can not be computed based solely on a qualitative object model. Only a significantly weaker functional model, kinematic topology, can be derived based on qualitative object models alone. This result means that at least in mechanical design, sketches do not represent a single qualitative model, but must be interpreted aa a set of possible precise models. Each step in the design process then refers to a particular precise model in this set. This novel interpretation of the use of sketches suggests alternatives to the popular model of conceptual design aa a symbolic mapping of functional into object attributes.