A spatial network is a network of 'spatial elements'. It is notoriously difficult to decide what a spatial element should be, and those used are informed by the application of the network analysis. In the theory of space syntax, Bill Hillier and Julienne Hanson use axial lines and convex spaces as the spatial elements. Axial lines are used to analyse urban networks, where the system generally comprises linear segments, whereas convex spaces are used to analyse building plans.