We present a two-part exploration on mobile true-3D displays, i.e. displaying volumetric 3D content in mid-air. We first identify and study the parameters of a mobile true-3D projection, in terms of the projection's distance to the phone, angle to the phone, display volume and position within the display. We identify suitable parameters and constraints, which we propose as requirements for developing mobile true-3D systems. We build on the first outcomes to explore methods for coordinating the display configurations of the mobile true-3D setup. We explore the resulting design space through two applications: 3D map navigation and 3D interior design. We discuss the implications of our results for the future design of mobile true-3D displays.