  Weyl then introduces his two-component spinor theory in Minkowski space. Since one of his aims is to adapt Dirac’s theory to the curved spacetime of general relativity, Weyl develops a theory of local spinor structures for curved  spacetime.[100]  He achieves this by providing a systematic formulation of  local tetrads or Vierbeins (orthonormal basis vectors). Orthonormal frames had already been introduced as early as 1900 by Levi-Civita and Ricci. Somewhat later, Cartan had shown the usefulness of employing local orthonormal-basis vector fields, the so-called “moving frames” in his investigation of Riemannian geometry in the 1920s. In addition, Einstein (1928) had used tetrads or Vierbeins in his attempt to unify gravitation and electricity by resorting to distant parallelism with torsion. In Einstein’s theory, the effects of gravity and electromagnetism are associated with a specialized torsion of spacetime rather than with the curvature of spacetime. Since the curvature vanishes everywhere, distant parallelism is a feature of Einstein’s theory. However, distant parallelism appeared to Weyl to be quite unnatural from the viewpoint of Riemannian geometry. Weyl expressed his criticism in all three papers (Weyl (1929b,c,d)) and he contrasted the way in which Vierbeins are employed in his own work with the way they were used by Einstein. In the introduction Weyl (1929b) says: 