In this paper we analyze how interaction develops in multi-device environments by distinguishing two layers: an interaction in the large layer defines an interactive experience across different devices and locations, where roles and tasks evolve and intertwine; an interaction in the small layer defines the actions done and the interaction techniques used to execute a specific, self-contained task on a device. We present a notation to describe interaction in the large and demonstrate how it can be useful to understand the interaction layers perceived by the users during an interactive experience. We finally report about a user experiment in the context of a real application scenario to evaluate concretely which interaction layers the users observe and if they are able to recognize the boundaries theoretically identified through the notation.