Previous research investigating sketching processes during object visualisation and design has indicated that drawing occurs in a primarily part-by-part manner, whereby the component structures of objects appear to dominate the organisation of ongoing activity. Some non-part-by-part sketching does arise, however, and appears to be closely linked with those parts of objects that possess multiple functionality. The present experiment aimed to provide further evidence to support the influence of functional knowledge on sketching. Overall, the results indicate that functional understanding is an important element of visual reasoning and sketch production in design-related tasks. We propose that functional knowledge serves simultaneously to promote certain aspects of representational accuracy (e.g., in terms of functional properties of parts) whilst, paradoxically, engendering other aspects of representational inaccuracy (e.g., in terms of the precise geometric structure of parts).