The general theory of relativity can only be formulated with the help of tensor calculus. Einstein became acquainted with this in 1912 in the form of absolute differential calculus. Its creator was Gregorio Ricci, who, along with Sophus Lie and others, succeeded in developing the theory of differential invariants. The absolute differential calculus fit the general theory of relativity like a key fits a lock: the calculus developed by Ricci between 1884 and 1892 precisely fulfilled the physical concept of the general theory of relativity that Einstein elaborated between 1907 and 1915. Such a correspondence was only possible because both Ricci within mathematics and Einstein within physics posed comparable questions, namely questions about invariants under specific transformations. An attempt is made to present the historical path as accurately as possible based on the sources.