Visual reasoning in design is facilitated by sketching. This research investigates how designers sketch, specifically analysing the physical details of mark making. It relates the graphical representations to the abstract cognitive processes of architectural design. A new form of protocol analysis has been developed using video and computer records of designers' sketching activity. The analysis of the resulting data compares the designer's retrospective commentary and interpretations of the sketching activity with the computer's record of that activity. The analysis will lead to a greater understanding of the relationships between 'Design Events' and 'Graphical Events' and thus how the notational activity of sketching supports the cognitive activity of conceptual design.