By the beginning of the twentieth century the new branch of mathematics, tensor calculus, was developed in the works of Gregorio Ricci Curbastro and Tullio Levi Civita of the University of Padua and the University of Rome "La Sapienza". Marcel Grossmann of the University of Zurich who had a deep knowledge of the Italian school of geometry and who was close to Einstein introduced to him these concepts. The collaboration between Einstein and Grossmann was essential for the development of General Relativity.