Considerable research has been done by various scholars to assess the significance of sketching in the early stages of the design process. However, sketching in design studies usually corresponds to drawing and the extensive research on the cognitive aspects of sketching does not always include three-dimensional sketching through physical and digital models produced in the early phases of design process. The aim of the presented research is to identify some characteristics of model-making that make it effective in the design process and design cognition as a form of sketching. Departing from key research on sketching which articulates its uncertain nature as a positive drive in early design phases, this thesis looks at whether physical and digital models can also be counted among ambiguous design tools. The inquiry is supported by empirical data from the protocol studies realized with three graduate students of architecture.