Zip Codes - History of
The number of items sent in the U.S. Mail delivery system increased substantially during the second half of the Twentieth Century, due mainly to business mail and advertisements. In 1962, an Advisory Board of the Post Office Department, made several recommendations. One was that the department develop a coding system, an idea that had been under consideration in the Department for a decade or more. Postmaster General John A. Gronouski announced that the ZIP Code would begin on July 1, 1963.
By that date, a five-digit code had been assigned to every address throughout the country." 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 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."
As of 1967, the Post Office required mailers of second- and third-class bulk mail to presort by ZIP Code.
source: History of the U.S. Postal Service.