Announced on April
30, 1963, and put into effect on July 1, ZIP—or Zoning Improvement Plan codes
gave the post office a better, more efficient way to sort mail. "The first
digit designated a broad geographical area of the United States, ranging from
zero for the Northeast to nine for the far West. This number was followed by
two digits that more closely pinpointed population concentrations and those
sectional centers accessible to common transportation networks. The final two
digits designated small Post Offices or postal zones in larger zoned
cities."
Source: http://mentalfloss.com/article/50450/50-things-turn-50-2013
