Nielsen divides the country into 210 DMAs. These areas represent 210 television media markets.
DMAs are more than just cities. For instance, the Philadelphia DMA encompasses not just the city, but all areas where Philadelphia stations are watched the most. The Philadelphia DMA includes southern New Jersey and most of Delaware.
Because of that, a Philadelphia TV station would want to cover news across the DMA and not just in the city. It would try to sell advertising to companies throughout the area.
Every county in the U.S. is in a DMA. Some DMAs cover a huge geographical area, like the Salt Lake City DMA, which stretches across the entire state of Utah. Others are geographically small, like the Providence DMA, because of all the nearby New England cities with their own stations.
The deciding factor in determining which DMA Nielsen assigns a county comes down to viewing habits. If more than 50% of homes in a county watch Baltimore TV stations, then the county is assigned to the Baltimore DMA. That's true even if the county is geographically closer to another city, like Washington, DC.
Occasionally, Nielsen will shift a county from one DMA to another. Maybe people in our example county suddenly get Washington, DC stations on their cable system and decide they'd rather watch the news from the nation's capital than from Baltimore. Once more than 50% of homes watch DC television more than Baltimore TV, Nielsen will move the county into the Washington, DC DMA.
Nielsen publishes an annual list of DMAs by audience size. Currently, there are 210 DMAs, with New York ranked number one and tiny Glendive, Montana at 210.