Large displays on smartphones accommodate tasks needing more screen real-estate, but at the expense of limiting the use of one-handed operations. In this paper we explore thumb reachability on an articulated dual-display smartphone. While mobile devices are featuring dual screens, no previous work has explored how each physical display’s positions impact one-handed reach and interaction. We explore 32 inter-display configurations resulting from the combination of orientations of the two displays along two axes. In a preliminary study we explore how users grasp such dual-display configurations. We then conduct a study to investigate the effects of the relative position of two displays on thumb reachability. Results provide a range of dual-display configurations that are ideal for one-handed use, enabling knowledge on how apps for such emerging devices can be optimized for thumb input.