Working with public census data, and the relatively new Zone Improvement Plan (ZIP) codes created by the Post Office, Robbin would produce his great masterpiece by 1978. He called it the “Potential Ratings in ZIP Markets” system, or PRIZM. PRIZM sorted the entire U.S. population into forty subnations, or “clusters,” each with a set of exact geographical locations. With PRIZM, a new reality revealed itself to Robbin: there was no United States, but forty distinctive nations all calling the same continent home.

Tim Wu, The Attention Merchants, loc. 604