The fact that there are so many different definitions shows the reality that rural and urban are multidimensional concepts. In simple terms: it's not easy to make clear-cut distinctions between the two. Is population density the defining concern, or is it geographic isolation? Is it small population size that makes it necessary to distinguish rural from urban? If so, how small is rural? Because so many people in the U.S. live in areas that are not clearly rural or urban, seemingly small changes in the way rural areas are defined can have large impacts on who and what are considered rural. As many agencies, researchers, and policymakers have discovered, the key is to use a rural-urban definition that best fits the needs of a specific activity, initiative, or effort.