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How the U.S. Postal Service Works

        Culture | Agencies

ZIP Codes
A mail carrier loads his truck in Modesto, Calif.
A mail carrier loads his truck in Modesto, Calif.
Justin Sullivan/Getty Images

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, ZIP codes became mandatory on all mail.

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.

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