The Post Office Department introduced the ZIP (Zone Improvement Plan) code system on July 1, 1963. The new ZIP code plan created a numerical representation of U.S. addresses aimed at speeding up mail processing. The five numbers represented areas of the United States and were used to direct mail through mechanized equipment in sorting centers.
The first digit indicates a geographical area of the country, beginning with zero in the northeastern states and ending with nine in the west. The next two digits indicate mail sorting centers near the final address. The last two digits represent post offices in small cities or postal zones in larger cities.
The public was initially reluctant to memorize and add numbers to their mailing and return addresses. Looking to make the service more user-friendly, the Department created the mascot of Mr. ZIP. The Department began a nation-wide publicity campaign for the service, featuring Mr. ZIP on posters and in radio and television advertisements. Cardboard cut-outs were placed in post offices around the country.
In 1983 officials added four more digits to ZIP codes allowing mail to be sorted even more specifically. These numbers allow automated mail processing machines to read and sort letters at speeds of 35,000 letters an hour.