What technique did John D. Rockefeller use to create a monopoly in certain markets?

John D. Rockefeller employed horizontal integration as a key technique to create a monopoly in the oil industry. This strategy involved acquiring or merging with other companies that produced the same product, which in Rockefeller's case was oil. By consolidating many smaller oil companies under the Standard Oil Company, he could control a significant portion of the market, effectively reducing competition and enabling him to set prices and influence the market more broadly.

This approach allowed Rockefeller to achieve economies of scale, where larger operations could lower costs and increase efficiency. His control over the refining process and supply chain further strengthened his monopoly, as he could dictate terms and streamline production to outperform competitors. As a result, horizontal integration was instrumental in establishing Standard Oil as a dominant force in the petroleum industry during the late 19th century.