In 1959–1972 economists laid down the theoretical foundations for applying marketable permits. These are sometimes seen as corollaries of the Coase Theorem. The theorem states that when two agents that impose an external effect on each other can negotiate its allocation in a costless way, they can agree on its socially justified quantity. However, it seems that the idea of permit marketability emerged before Coase (who was honoured with the Nobel prize in 1991) formally stated his theorem.