Public display screens are relatively recent additions to our world, and while they may be as simple as a large screen with minimal input/output features, more recent developments have introduced much richer interaction possibilities supporting a variety of interaction styles. In this paper we propose a framework for classifying public display systems with a view to better understanding how they differ in terms of their interaction channels and how future installations are likely to evolve. This framework is explored through 15 existing public display systems which use mobile phones for interaction in the display space.