  In 1918, Weyl proposed such a theory. In Weyl (1918a, 1919a), and in  the third edition (1920) of Raum-Zeit-Materie, Weyl  presented his ingenious attempt to unify gravitation and  electromagnetism by constructing a gauge-invariant geometry (see  below), or what he called a purely infinitesimal  ‘metric’ geometry. Since the conformal structure  \(C\) (see below) of spacetime does not determine a unique  symmetric linear connection \(\Gamma\) but only an equivalence class  \(K = [\Gamma]\) of conformally equivalent symmetric linear  connections, Weyl was able to show that this degree of freedom  in a conformal structure of spacetime provides just enough room for  the geometrization of the electromagnetic potentials. The resulting  geometry, called a Weyl geometry, is an intermediate  geometric structure that lies between the conformal and Riemannian   structures.[46] 