ABSTRACT
The five-digit numbering code and sorting system known as zip code—for “zone improvement plan”—was implemented by the United States Postal Service in 1967 to streamline mail delivery and speed customer service and to improve the sorting of third-class bulk-rate mail. The number sequence arranges addresses against a numerical list, beginning with 00000 and progressing through 99999. Each zip code refers to a specific geographic mail delivery area. A single unique zip code can cover an area within a state or can represent a single company or building that receives a high volume of mail. In rare instances, a zip code area can cross state borders. A nine-digit numbering system, referred to as zip+4, was introduced in 1983, adding the extra numbers to designate carrier route to further improve delivery services to businesses.
