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So what do the extra numbers mean? These last 4 digits represent specific delivery routes within delivery areas. This extra detail means an even more precise matching to a more granular level. It could represent a handful of houses on one side of a street, or even a single building that receives a high volume of mail.

Primary state prefixes. ZIP Codes are numbered with the first digit representing a certain group of U.S. states, the second and third digits together representing a region in that group (or perhaps a large city) and the fourth and fifth digits representing a group of delivery addresses within that region.

A United States postal code, for example, might be either five digits or five digits followed a hyphen (dash) and another four digits (12345-1234).

This is because ZIP code boundaries are based on mail routes and delivery points rather than geographic factors. The U.S. Post Office Department, predecessor to USPS, developed a zoning address system in 1943 as a way to make sorting and delivery of mail easier.

It is preferred that all City, State, and ZIP Code information be on a single line. If that is not possible, the ZIP Code, including the ZIP+4 Code, may be placed below the city/state information.

The standard ZIP code notation used by the United States Postal service uses five digits to identify a delivery area. An example of a standard US ZIP code is 90210.

A 5-Digit ZIP code represents a USPS delivery area. A 9-Digit ZIP code, also known as a ZIP+4 code or an extended ZIP code, includes the same information as the 5-Digit ZIP code, plus additional information about a specific delivery route within the delivery area.