

Hence, in the market for local telephone services, there is a need for only one firm competition will not naturally arise. The telephone company's long‐run average costs may eventually rise but only at a level of output that is beyond the level the local market demands. For example, a local telephone company's marginal and average costs tend to decline as it adds more customers as the company increases its network of telephone lines, it costs the company less and less to add additional customers. A few monopolies arise naturally, in markets where there are large economies of scale. Not all monopolies arise from these kinds of barriers to entry. The market for diamonds, for example, is dominated by a single firm that owns most of the world's diamond mines.

Second, there are high barriers to entry. First, there is only one firm operating in the market.

Three conditions characterize a monopolistic market structure. Hence, in a monopolistic market, there is no difference between the firm's supply and market supply. This one firm provides all of the market's supply. In contrast, in a monopolistic market there is only one firm, which is large in size. In a perfectly competitive market, there are many firms, none of which is large in size. Labor Demand and Supply in a Perfectly Competitive Market.Equilibrium in a Perfectly Competitive Market.Monopolistic Competition in the Long-run.Demand in a Perfectly Competitive Market.Classical and Keynesian Theories: Output, Employment.
