In the United States, a 5-digit ZIP Code is a postal code used by the United States Postal Service (USPS). It consists of a five-digit code plus four additional digits that determine a more specific location within a given ZIP Code. The basic format consists of five numerical digits. An extended ZIP+4 code was introduced in 1983 which includes the five digits of the ZIP Code, followed by a hyphen and four more digits that reference a more specific location.
The first digit of the ZIP Code indicates whether it is east or west of the Mississippi River. A zero (0) denotes the eastern United States, while a one (1) denotes the western United States including Alaska and Hawaii. As of 1963, only thirteen states are entirely west of the Mississippi: Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming. The second and third digits indicate region within these large areas; generallythe lower Midwest is denoted by central zip codes beginning with 0 or 1 while coastal regions have zip codes starting with 2 or 3 and mountain/desert regions have those starting with 4 or 5 respectively.
The fourth and fifth digits indicate small towns or delivery routes within this region; originally almost all mail had to be delivered to Post Offices where sorting into local deliveries was done manually but since automation most large businesses now have their own direct mail receiving addresses so many letters carry an extra barcode identifying both destination address and Post Office for automated sorting purposes instead.