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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.

The most complete ZIP Code is a nine-digit number consisting of five digits, a hyphen, and four digits, which the USPS describes by its trademark ZIP+4. The correct format for a numeric ZIP+4 code is five digits, a hyphen, and four digits.

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.

If you want to improve the chances that the letter will make it to its destination quickly, you should use the five-digit ZIP code plus the four additional numbers that are specific to the area. To format, write the ZIP code with a hyphen afterward and then the four additional numbers will immediately follow that.

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.

There may be an extended code (ZIP+4), where a hyphen and a further four digits are added to the five-digit code. If you have the full 9-digit ZIP code you can include this, however, the first five digits are usually adequate.

The code became a tool for organizing and displaying demographic information while also allowing for the efficient sorting and dispatching of mail. Over the span of 50 years, the ZIP Code was expanded from 5 digits to 9 and then to 11, with each digit indicative of the address location.