Final answer:
The Standard Oil Trust created a monopoly by controlling stocks of various oil companies and making decisions to the benefit of Standard Oil's stockholders, ultimately controlling 90 percent of oil refineries in the U.S. through aggressive tactics like horizontal integration and special agreements with railroads.
Step-by-step explanation:
The Standard Oil Trust helped create a monopoly for Standard Oil by implementing a new corporate structure devised by John D. Rockefeller. Rockefeller's strategy involved the formation of a trust, where a small group of investors led by him acquired the stocks of various oil companies. These stocks were held 'in trust,' giving Rockefeller and his associates control over the companies' decisions and profits, without directly owning them, thus avoiding the legal definitions of monopoly at the time.
Horizontal integration was a significant part of Rockefeller's approach. He expanded Standard Oil's holdings by merging with competitors or driving them out of the market through underpricing, which, along with special deals with railroads for discounted shipping rates and purchasing competitors' stocks above market value, eliminated competition effectively. By the 1890s, this led to Standard Oil controlling 90 percent of the nation's oil refineries.
After legal challenges, the trust model was replaced by a holding company model, which continued to centralize power and effectively maintain the monopoly. Over time, other industries adopted Rockefeller's trust method, leading to widespread monopolies across various sectors until federal legislation like the Sherman Anti-Trust Act began to address the issue.