We present a novel approach to understand geometricincompatibility-induced rigidity in under-constrained materials, including sub-isostatic 2D spring networks and 2D and 3D vertex models for dense biological tissues. We show that in all these models a geometric criterion, represented by a minimal length min , determines the onset of prestresses and rigidity. This allows us to predict not only the correct scalings for the elastic material properties, but also the precise magnitudes for bulk modulus and shear modulus discontinuities at the rigidity transition as well as the magnitude of the Poynting effect. We also predict from first principles that the ratio of the excess shear modulus to the shear stress should be inversely proportional to the critical strain with a prefactor of three, and propose that this factor of three is a general hallmark of geometrically induced rigidity in under-constrained materials and could be used to distinguish this effect from nonlinear mechanics of single components in experiments. Lastly, our results may lay important foundations for ways to estimate¯ min from measurements of local geometric structure, and thus help develop methods to characterize large-scale mechanical properties from imaging data. M.M., B.P.T., and M.L.M. designed the research, M.M. performed the research and analyzed the data, K.B. provided important simulation data, M.M., B.P.T., and M.L.M. wrote the paper.The authors declare no conflict of interest.