Between the 1920s and the 1970s power plants and lines were built to supply electricity and were typically vertically integrated – meaning the utility owned the generation, transmission, and distribution. A key to keeping electricity rates low was due to utilities sharing reserve margins with others. In the case of outages, generators had to ensure reliable backup power and sharing those reserves with neighboring utilities through interconnections or power pools cut costs significantly. Power pools formed during this time included PJM, New York, New England, ERCOT, and SPP.