From "The State of Small and Rural Libraries in the United States", Institute of Museum and Library Services, September 2013.
Small library defined. In this analysis, we have defined small library based on the population of the library's legal service area. Libraries with a legal service area population of 25,000 or less were categorized as small libraries. After identifying all libraries below this threshold, these small libraries were further subdivided into the following categories:
Rural library defined: This analysis makes use of the urban-centric locale system of the Public Libraries Survey (PLS), which separates rural libraries into three categories.