Sketching is one of the most important activities in the design and development of new products. Designers produce sketches, from concept design ideas through to highly detailed representations of product artifacts, product sub-assemblies and so on, to communicate with themselves and with others. To this end, the focus of this paper is the description of a study of student designers at work in the early stages of design, with particular emphasis on the visible sketching component of the design process. The study has been carried out to firstly investigate the sketching activity and secondly to explore methods or techniques which might improve the efficiency of this activity. In particular, the paper describes a technique, based upon three types of operation, namely lateral transformations, vertical transformations and duplication, that occur between designer's sketches. These transformations can be used to help track the designer's thinking mode which, it is envisaged, will increase the efficiency of the sketching activity.