Friday, May 27, 2011


Zipology 101
by Professor Larry Shafer, Retired,
Cape Girardeau, MO

Until 1963, mail in the U.S. was simply addressed to the city and state. This was an efficient system until mail volumes began overwhelming the system.

That year, the Zone Improvement Program (ZIP) was introduced. It was an improvement over a smaller numerical system that some large cities were already using.

What does each number of the ZIP Code represent?
Digit one — The United States is divided into 10 regions. The New England area was designated as Region 0 and then the rest of the country is broken down from east to west numbering 1-9.

Digits two and three — Each region is broken down into sections that comprised the next two numbers in the ZIP Code, so mail could go to sectional centers.

Digits four and five — Finally, each office within the section is given a number that was represented in the last two digits. Usually, the largest office in the section is usually designated as the first, or 01, office in that sector. The others within that section are alphabetized and given successive numbers.

Digits six through nine — This worked reasonably well, but mail volumes continued to grow. Eventually, the simple five-digit code could no longer meet the demands of the service. In 1983, the nation’s addresses were broken down to an even smaller level and four additional numbers assigned. This was accomplished by taking every block face in a given city and assigning it a “ZIP+4” segment code. Additionally,  aapartment buildings, office building floors, and cluster boxes each have their own unique 4-digit code.

Eventually, two more digits were added, which allows Delivery Point Sequencing to sort the mail directly to
the address.

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