A merger occurs when two approximately equal-sized companies combine to form a single new company. Companies usually merge to be able to accomplish things together that they could not do individually. There are three main types of mergers: horizontal mergers between companies in the same industry, vertical mergers between companies in adjacent business stages (e.g. manufacturer and supplier), and conglomerate mergers between unrelated industries. Mergers allow companies to gain economies of scale through cost savings from increased size and production, but large integrated firms can also face diseconomies of scale like slow decision making.