ESRI Data & Maps 2005 : U.S. Three-Digit ZIP Code Areas

The first digit of a five-digit ZIP Code divides the country into 10 large groups of states numbered from 0 in the Northeast to 9 in the far West. Within these areas, each state is divided into an average of 10 smaller geographical areas, identified by the second and third digits.

These digits, in conjunction with the first digit, represent a sectional center facility or a mail processing facility area. These areas are serviced by the U.S. Post Office Sectional Center Facility (SCF). Note that a single SCF often services multiple three-digit areas.