Buzz
Most people are familiar with their five-digit ZIP code and maybe a few others in their area, but what’s the deal with the additional four digits that occasionally appear on mail?
Let’s begin with the five-digit codes you’re accustomed to. Back in the early 1960s, the U.S. Postal Service realized that the old Postal Delivery Zone System was no longer sufficient for managing rising mail volumes and the rapid urban and suburban growth. To address this, they introduced the Zone Improvement Plan (ZIP) in 1963. Each address received a five-digit code: the first digit represented a broad geographic region or state grouping (for instance, ‘1’ includes New York, Pennsylvania, and Delaware), the next two digits referred to a specific area or large city within that region (such as ‘91’ for Philadelphia), and the final two digits identified a smaller delivery zone or cluster of addresses within the region.
As the system continued to face pressure in the following years, the Postal Service modified it in 1983 to introduce the ZIP+4 system, which added four extra digits to the original code. These extra digits helped identify particular locations, such as apartment complexes, office buildings, or high-volume mail recipients, to streamline mail sorting and delivery. The sixth and seventh digits represented a 'delivery sector,' such as a set of streets, P.O. boxes, buildings, or even a single skyscraper. The eighth and ninth digits referred to a 'delivery segment,' which could pinpoint specific areas like one side of a street, a particular floor in a building, or even a designated department within a large office building.
Convincing the public to adopt the standard ZIP codes was already a challenge—some people were frustrated by the extra number to remember alongside their telephone area codes and Social Security numbers, while others felt that being assigned a number was impersonal and un-American. As a result, the ZIP+4 system never gained widespread public acceptance. Fortunately, when the expanded codes were introduced, advancements in technology meant that people didn’t need to memorize or use the full code. The USPS’s automatic sorting systems now apply a Postnet barcode to mail that corresponds to the complete ZIP+4 code, and multi-line optical character readers can easily interpret the barcode and the written address to determine the correct ZIP+4.
