designations that serve as guides to advertisers and advertising agencies in the preparation of media plans, first used by the original ACNielsen Company, to indicate counties in the United States by size of population (based on the most recent Census) and proximity to a central city or metropolitan area. All counties in the United States have only one designation.
A County: any county located in the 25 largest U.S. cities or their consolidated statistical urban areas.
B County: any county not designated as an A County that has population over 150,000 or is part of a consolidated statistical area with population over 150,000.
C County: any county or consolidated statistical area not designated as an A or B County that has population over 40,000.
D County: any county statistical area not designated as an A, B, or C County.

The consolidated areas are determined by the Federal Government Office of Management and Budget and are considered to be standard.