The Zone Improvement Plan (ZIP) was implemented by the United States Postal Service in the 1960s as an impovement on a previous, less efficient postal code system, the old system used mainly for large cities. The ZIP code system divides the country in to ten zones, numberd 0 to 9, east to west and north to south. Each of these zones is divided into 100 smaller areas. Each of these zones is further divided into the areas served by each post office, usually with a separate ZIP code number for street addresses and POBox addresses. This completes the basic five-digit postal code.
The extended (nine digit) postal code (ZIP+4), implemented in 1983, adds four digits to divide each zone into smaller areas, but the additional digits remain optional in addressing postal mail; only the basic five-digit ZIP code is mandatory.
