Tabletop systems provide a platform for developing novel interaction systems, including tangible user interfaces (TUIs). This paper presents a study of the effects of a tabletop system with tangible user interfaces on designers’ spatial cognition and design communication in collaborative design. In devising an experiment that can highlight the impact on spatial cognition while using TUIs, we compared designers using a tangible user interface (TUI) on a tabletop system to designers using a graphical user interface (GUI) on a typical desktop computer with mouse and keyboard. The designers were given a configuration design task in which they manipulated 3D objects to meet design specifications. Our preliminary findings are that designers using the tabletop system with TUIs reasoned about spatial relationships among 3D objects and discovered unexpected spatial relationships, while the designers using the traditional keyboard and mouse interfaces reasoned about individual 3D objects.