The paper deals with Maffeo Pantaleoni's (1857-1924) analysis of money and banking and shows that the Italian economist developed one and the same theory for both, whose central feature was the notion of flexibility and whose main claim was that the most important function of money and banks is the loosening of the real and temporal constraints caused to production and exchange activities by the complementarity relationships among production factors. This theory is crucial to understand how Pantaleoni turned from the “pure” equilibrium theory of the 1889 Principii to the “impure” theory of his later essays, where no general or partial equilibrium may arise.