From the historical point of view, no doubt the work of Tullio Regge can be seen as a first attempt to develop a model out of which geometrical concepts could be developed. The original paper dates from 1961, see Regge (1961). More specifically, we are concerned here with general relativity theory (GRT). Although the original intention of Regge was to construct techniques for solving the equations of GRT in the “difficult” cases, i.e., where there is no symmetry present and perturbation theory is not applicable. Rather than transcribing the differential equations of GRT into difference equations, Regge looked for a technique that leads to different equations altogether. Without presenting the full details, the core concept of his approach is the “deficit angle”. In GRT we are dealing with curved spaces. Take a two-dimensional curved surface. If it is flat, then it can be covered with triangles. If it is curved, it can be approximated with triangles, but with an important difference. Suppose that triangles meet at vertices, then we can look at one particular point and all the triangles that meet in that point. If that part of the surface is flattened out, there will be a gap somewhere. To this gap corresponds an angle and that is precisely the deficit angle. The larger the curvature, the larger the deficit angle. The same technique works for the four-dimensional case, where instead of triangles, simplexes are used. The beauty of this approach is that the equations of GRT can be rewritten in terms of deficit angles and lengths of the edges of the simplexes and solved in terms of these concepts. Misner et al. (1973) contains a chapter (42: “The Regge Calculus”) that explains Regge’s approach in a compact and perfectly accessible way.