Oligopolists usually try not to engage in price cutting, excessive advertising or other forms of competition. Thus, there may be unwritten rules of collusive behavior such as price leadership (tacit collusion). A price leader will then emerge and it sets the general industry price, with other firms following suit. For example, see the case of British Salt Limited and New Cheshire Salt Works Limited. Classical economic theory holds that Pareto efficiency is attained at a price equal to the incremental cost of producing additional units. Monopolies are able to extract optimum revenue by offering fewer units at a higher cost.