From the 1930s to the early 1960s, the volume of mail -- particularly business mail -- grew significantly, and the need for a better system became apparent. On July 1, 1963, the USPS introduced the ZIP code (Zone Improvement Plan) system. In 1967, the ZIP codes became mandatory on all mail.
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A mail carrier loads his truck in Modesto, Calif.
A ZIP code is a five-digit number representing a specific location in the United States. The extended ZIP + 4 code adds a hyphen and four additional digits for an even more precise location. Here is how it works:
- The first digit represents the state. Numbers increase as you move west. Several states share each digit -- 2, for example, represents the District of Columbia, Maryland, North Carolina, South Carolina, Virginia and West Virginia.
- The second and third digits represent regions within the state -- the first three digits create the Sectional Center Facility (SCF) code. SCFs are the regional headquarters for mail sorting and distribution.
- The fourth and fifth digits represent more specific areas, like post offices and postal delivery zones within a city or town.
- ZIP + 4 has four extra digits that identify a specific segment of the five-digit delivery area -- like a city block, office building or individual high-volume mail receiver.