Regulation is a substitute for free market operation typically, in the gas industry, where all or part of the operations are carried out by institutional or natural monopolies. Provides a temporary or permanent counterweight to the economic power of the monopoly. Often exercised politically by government departments, but in many countries independent regulation is thought preferable. In any case the rules governing regulation will normally have been laid down explicitly by government legislation. Regulation can be economic – restricting the revenue and profits of operators - or legal – policing legislation. Regulators are normally closely involved in the setting up of liberalised market systems, when they may use their central position to interpret and rule on the applicability of legislation either ex-ante or ex-post. See Common Carriage.