In this work we propose a set of guidelines for tangible interfaces (TUI) for mapping physical objects and digital functions. These guidelines emphasize an aspect that has been only partly analyzed so far in TUI research: the role of the object shape as a means for suggesting how to manipulate the artifacts. The peculiarity of the proposal - general enough to be used for different application domains - is the mapping to discrete and continuous digital functions based on the type of surfaces that define the physical artifacts of the interface. The application of the guidelines to a widely diffused task, the query and navigation of a multimedia database, and its evaluation with a class of students gave us interesting insights about the appropriateness of the mapping and its learnability.