High Quality Content by WIKIPEDIA articles! An inversive plane is a class of incidence structure in mathematics.A figure may look like a graph, but in a graph an edge has just two ends (beyond a vertex a new edge starts), while a line in an incidence structure can be incident to more points. An incidence structure has no concept of a point being in between two other points; the order of points on a line is undefined. Compare with ordered geometry, which does have a notion of betweenness.