Monopoly
A monopoly can be formed by the government, by merging a few firms to form a large one, or form naturally to become a natural monopoly. In many places, utilities such as telephone service or cable television are monopolies, as it is very costly to build the infrastructure for another firm to compete in the market. Such natural monopolies is hence allowed by the government, but is usually strictly controlled to prevent the monopoly from charging a very high price for a product or service.