It is widely accepted that automatic interpretation of drawings and documents will not be achieved in the short to medium term and that manual digitisation is not an acceptable alternative. Despite this there has been little systematic discussion of the role of the user in line drawing interpretation. We consider desirable properties of human interaction with line drawing interpretation systems, argue that descriptions are required of the inference structure of line drawing interpretation tasks and systems if appropriate and effective user involvement is to be achieved, and develop and compare the inference structures of two well-developed line drawing interpretation systems; one automatic, one interactive. Analysis of these representations highlights two areas in which interactivity could be improved. An interactive system addressing one of those areas is outlined and directions for future research are identified.