How do monopolistic competition and oligopoly differ from other market structures?

"Three characteristics define the market structure known as monopolistic competition:\n\n• Many firms are in the market.\n\n• The firms offer products that are close substitutes but still differ from one another.\n\n• It is relatively easy to enter the market.\n\nUnder monopolistic competition, firms take advantage of product differentiation. Industries where monopolistic competition occurs include clothing, food, and similar consumer products. Firms under monopolistic competition have more control over pricing than do firms under perfect competition because consumers do not view the products as perfect substitutes. Nevertheless, firms must demonstrate product differences to justify their prices to customers. Consequently, companies use advertising to distinguish their products from others. Such distinctions may be significant or superficial. For example, Nike says “Just Do It,” and Tylenol is advertised as being easier on the stomach than aspirin.\n\nAn oligopoly has two characteristics:\n\n• A few firms produce most or all of the output.\n\n• Large capital requirements or other factors limit the number of firms.\n\nBoeing and Airbus Industries (aircraft manufacturers) and Apple and Google (operating systems for smartphones) are major players in different oligopolistic industries. With so few firms in an oligopoly, what one firm does has an impact on the other firms. Thus, the firms in an oligopoly watch one another closely for new technologies, product changes and innovations, promotional campaigns, pricing, production, and other developments. Sometimes they go so far as to coordinate their pricing and output decisions, which is illegal. Many antitrust cases—legal challenges arising out of laws designed to control anticompetitive behavior—occur in oligopolies.\n\nThe market structure of an industry can change over time. Take, for example, telecommunications. At one time, AT&T had a monopoly on long-distance telephone service nationwide. Then the U.S. government divided the company into seven regional phone companies in 1984, opening the door to greater competition. Other companies such as MCI and Sprint entered the fray and built state-of-the-art fiber-optic networks to win customers from the traditional providers of phone service. The 1996 Telecommunications Act changed the competitive environment yet again by allowing local phone companies to offer long-distance service in exchange for letting competition into their local markets. Today, the broadcasting, computer, telephone, and video industries are converging as companies consolidate through merger and acquisition."