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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.

What is the four-digit extension? It represents a specific delivery route within that overall delivery area. For PO boxes, the four-digit extension usually includes the PO box number. Including the last four digits means mailings will arrive to their intended destination faster, as well as discounts on bulk mailings.

Since our caller works for the U.S. government, and ZIP codes are a government thing, I checked the US Printing Office style guide first. It recommends ?ZIP Code.? Associated Press style is similar: ?ZIP code??two words with ?ZIP? in all caps?but it uses a lowercase C, and I was surprised it would deviate.

Unknown, empty, or otherwise invalid integers are represented by 0. Zip codes will require leading zeros to correctly format 5-digit and zip-plus-4 codes. Zip-plus-4 notation using a plus-4 of all zeros (-0000) is not valid - use the 5-digit instead.

A 5-digit number that identifies a particular postal delivery area in the United States.

Initially, the zip code was only a two-digit number: the first denoted the city, the second denoted the state. But as the need for delivery expanded, so did the concept of the zip code. As of 1963, zip codes' numbers are determined by a few factors: the area, the regional postal facility and the local zone.

ZIP & ZIP+4 United States ZIP codes are five numerical digits long. ZIP+4 codes, also known as add-on or plus-four codes, include four additional digits that identify a geographic segment within a ZIP code delivery area. The first digit in a ZIP code represents a defined group of U.S. states.