After Einstein and Grossmann had tested and then rejected a number of candidates for a new theory of gravity, they publish a theory that is no longer valid from today’s perspective, the Entwurf Theory. Nevertheless, its mathematical elaboration creates the prerequisites for formulating the correct theory in late 1915, and for demonstrating its consequences for physics. The difference between the Entwurf Theory and the General Theory of Relativity is that the former gives priority to certain reference systems.