We identify and present a major interaction approach for tangible user interfaces based upon systems of tokens and constraints. In these interfaces, tokens are discrete physical objects which represent digital information. Constraints are confining regions that are mapped to digital operations. These are frequently embodied as structures that mechanically channel how tokens can be manipulated, ... read moreoften limiting their movement to a single degree of freedom. Placing and manipulating tokens within systems of constraints can be used to invoke and control a variety of computational interpretations.We discuss the properties of the token+constraint approach; consider strengths that distinguish them from other interface approaches; and illustrate the concept with eleven past and recent supporting systems. We present some of the conceptual background supporting these interfaces, and consider them in terms of Bellotti et al.'s  five questions for sensing-based interaction. We believe this discussion supports token+constraint systems as a powerful and promising approach for sensing-based interaction.read less
Ullmer, Brygg, Hiroshi Ishii, and Robert J.K. Jacob. "Token+constraint systems for tangible interaction with digital information." ACM Transactions on Computer-Human Interaction 12 no. 1 (March 2005): 81-118. doi: 10.1145/1057237.1057242.