A Metropolitan Statistical Area (MSA) is a county or group of counties that also has a central city of at least 50,000 inhabitants. Contiguous counties are included in the metropolitan statistical area if they have a certain level of social and economic interaction with the population nucleus. In the figure, Byzantium represents a one-county MSA for the city of Shaney.
MSAs, unlike counties and their equivalent, do not cover the entire geography of the United States. MSAs are reserved for densely populated regions only. Moreover, they often overlap state boundaries. For example, the MSA for the Washington D.C. area comprises parts of three states (or state equivalents) — Maryland, Virginia, and D.C.
Although MSAs, where they exist, often overlap state boundaries, they are nevertheless faithful to county boundaries; and where MSAs don't exist, counties still function as the primary division of small area geography.