  After Einstein applied Riemannian geometry to his theory of general  relativity, Riemannian geometry became the focus of intense research. In particular, G. Ricci and T. Levi-Civita’s so-called Absolute  Differential Calculus developed and clarified the Riemannian  notions of an affine connection and covariant  differentiation. The decisive step in this development, however, was T. Levi-Civita’s discovery in 1917 of the concept of  infinitesimal parallel vector displacement, and the fact that such parallel vector displacement is uniquely determined by the metric field of Riemannian geometry. Levi-Civita’s construction  of infinitesimal parallel transport on a manifold required the  process of embedding the manifold into a flat higher-dimensional  metric space. In 1918, Weyl generalized Levi-Civita’s concept of  parallel transport by means of an intrinsic construction  that does not require the process of such an embedding, and is  therefore independent of a metric. Weyl’s intrinsic construction  results in a metric-independent, symmetric linear  connection. Weyl simply referred to the latter as  affine connection.[38] 