St. Louis, San Francisco, and Washington, D.C. Notes: The B and C cities are secondary because they are in the same market area as a city rated A. While those are only abstract examples of the ratings, consider how the system works in practice with the example of Ohio. Rand McNally conceives of Ohio as consisting of nineteen markets (trading areas) with some peripheral counties being contained in trading areas of other states. The table below shows the nineteen trading areas in order of hierarchy. Each has an A-rated city. Some also have other rated cities and suburbs. Example: Ohio The system is applied by businesses seeking to serve a network of customers with the greatest efficiency. For example, a company that services elevators or a wholesaler of paper bags would want to locate its field representatives primarily in A rated cities. The Rural Health Research Center at the University of Minnesota published a report in 2008 on access to physicians in rural areas which uses the system to evaluate access to physicians in rural areas. See also Metropolis Megalopolis (city type) Primate city Financial centre Ecumenopolis Global City References Rand McNally Commercial Atlas and Marketing Guide, 136th edition, 2005, Skokie, Ill.: Rand McNally. A complete description of the system is found at Volume 1, pp. 7–8, and rosters of the ratings appear on pp. 74–87 and 129-131. Richard L. Forstall, ed. Rand McNally city rating guide: a completely new concept in analyzing markets and planning sales and advertising programs in the 1090 principal business centers of the United States. Chicago: Rand McNally, 1964. Cities in the United States Rating systems