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Dates When Postal Zones and Zip Codes Started
POSTAL ZONES - You may have noticed that many addresses during the period between 1943 and 1963 had a one or two digit number following the city name. These numbers were postal zones. It may surprise you to learn that postal zones were instituted in 1943 during WWII. They were necessary because many postal clerks had gone into the service and the new inexperienced postal clerks were having trouble sorting the mail. The zone system was put in place to make things easier.

ZIP CODES - By 1963, most of first-class mail in the United States was generated by a small number of large-volume mailers, so The Post Office Department devised a plan to speed handling and delivery of letter mail. By this time most businesses had automated mailing systems that could easily handle the 5 digits that would allow mailings to bypass as many as six mail-handling steps. Zip codes went into effect on July 1, 1963. ZIP stood for Zone Improvement Plan.