Buying and taking over another company that sells the same products
Horizontal integration is similar to vertical integration. While vertical integration involves buying other companies that help to supply your own business, horizontal integration involves buying a direct competitor.
Horizontal integration can also lead to a monopoly, which occurs when one company controls the entire market for a single product.
Rockefeller used horizontal integration to expand his Standard Oil business. He bought other oil companies, and eventually Standard Oil refined about 90 percent of the country's oil.
-Jack Schooley, John O'Donnell, Matthew Marceau