  When Einstein first introduced his general theory of relativity in the 1910s, its novel feature was its general covariance. It was the first spacetime theory in which one was free to use arbitrary spacetime coordinate systems.  This feature is now shared by virtually all modern formulations of spacetime theories, including modern versions of special relativity and Newtonian spacetime theory.  In its original form, general covariance was understood “passively”; that is, as a freedom to describe structures in spacetime by means of arbitrarily chosen coordinate systems. That freedom proves to be equivalent to another freedom, known as “active” general covariance.  According to active general covariance, we are licensed to spread geometrical structures like metrical fields over the manifold in as many different ways as there are coordinate transformations. (For a more extensive account of the relationship between active and passive covariance, see the supplementary document: Active and Passive Covariance.)[2]