Location information can be used to enhance interaction with mobile devices. While many location systems require instrumentation of the environment, we present a system that allows devices to measure their spatial relations in a true peerto-peer fashion. The system is based on custom sensor hardware implemented as USB dongle, and computes spatial relations in real-time. In extension of this system we propose a set of spatialized widgets for incorporation of spatial relations in the user interface. The use of these widgets is illustrated in a number of applications, showing how spatial relations can be employed to support and streamline interaction with mobile devices. ACM Classification H5.2 [Information interfaces and presentation]: User Interfaces. - Graphical user interfaces. General Terms Design, Human Factors