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US ZIP Codes

United States ZIP codes are postal codes that are used to assist the USPS (United States Postal Service) route mail in a quick and efficient manner. ZIP codes are also commonly referred to as postal codes. The abbreviation ZIP stands for Zone Improvement Plan. The 5 digit format was popularized in 1963, however the additional 4 digits after the dash was later introduced into the system (zip+4 digit code).

These additional digits better assist the United States Postal Service to more accurately group mail documents for delivery. The USPS originally created the ZIP code system, however other shipping services like UPS and FedEx utilize these Zip codes for regulating, calculating, and sorting packages along with precisely monitoring shipping rates.

Types of Zip Codes

  • High volume addresses (example - 20524 - FBI in Washington, DC)
  • PO Box (example - 35241 for the PO Box of Norfolk, VA)
  • Military ZIP codes
  • Standard ZIP codes (everything else)

Assignment of ZIP codes and Mail Sorting

Within the zip code structure, the first digit of a US ZIP code represents a group of US states. The main purpose of ZIP codes is efficiency for the delivery of US mail. Some codes cover multiple states to ensure mail is routed as well as delivered in an effective manner. The next 3 digits of a ZIP code help determine the mail facility (AKA – sectional center facility or sec center) which is primarily used to sort and process all mail within that particular state. The last 2 digits represent where the mail will be distributed according to the local post offices. Sectional center facilities are never open to the public and sort all mail documents during the night hours.

The additional 4 digits after the zip code is never required, but certainly helps aid the postal office with additional mail sorting. These 4 digit numbers, followed by the zip code correspond to high volume receivers, apartment complexes, or city blocks. It’s very common for a PO Box to correspond to a zip code along with these appended 4 digits. This is not a universal rule however, zip codes + 4 digits must be confirmed for each PO Box address.