Barriers to entry are critical factors that prevent new firms from entering a market, thereby protecting established companies. One significant barrier is the ownership of key resources. A prime example is De Beers, which historically controlled a vast majority of the world's diamond mines. This control made it exceedingly difficult for new entrants to access the essential resource of diamonds, effectively blocking their entry into the diamond industry.

Another barrier arises from government regulation, particularly through the patent system. When an inventor creates a new product, they can file for a patent, granting them exclusive rights to produce and sell that product. This legal protection means that competitors cannot legally enter the market with the same product, thus maintaining the inventor's monopoly and preventing others from capitalizing on their innovation.

Economies of scale also play a crucial role in shaping market structures, particularly in oligopolies. Economies of scale refer to the cost advantages that firms experience as they increase production. As a company produces more units, the average total cost per unit decreases due to factors such as specialization and bulk purchasing discounts. In a graph illustrating long-run average total costs, the curve for perfect competition quickly reaches its minimum efficient scale, indicating that many small firms can operate at low quantities to meet demand. In contrast, an oligopoly can sustain lower average total costs over a larger output, allowing fewer firms to meet higher demand efficiently.

This scenario can lead to a natural duopoly, where only two firms dominate the market due to the significant economies of scale. In such cases, if two firms each produce a substantial quantity, they can effectively satisfy the market demand without the inefficiencies that would arise from numerous smaller competitors. As we explore further into the dynamics of oligopolies, the focus will primarily be on these duopolies, as they provide a simplified yet insightful framework for understanding market behavior.