The United States Post Office Department first implemented postal zones for in 1943. World War II caused thousands of experienced postal employees to leave and serve in the military. To offset this loss, the Post Office Department began a zoning address system in 124 cities so that mail could be separated by employees who didn’t have the same knowledge and experience.
By 1963, computers brought centralization of accounts and mass amounts of mail were being sent in the form of bills, bank deposits, advertising, magazines, mortgage bills, and payments, Social Security checks and more. Due to this volume of mail, the Post Office needed to advance its methods to be as efficient as possible. What resulted was the Zoning Improvement Plan (ZIP) Code, which launched on July 1, 1963.
ZIP codes are five digits long. The first digit is assigned to a broad geographical area of the United States, ranging from zero in the Northeast to nine in the West. The next two digits pinpoint population concentrations and sectional centers accessible to common transportation networks. The final two digits in the code designate small Post Offices or postal codes in larger zoned cities.
There are approximately 42,000 zip codes in the United States today.