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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.

In 1983, the zip + 4 format we now use (e.g., 60601-1635) was introduced to support delivery to densely populated areas. The first three digits before the hyphen identify the sectional center or large city. The second two classify the post office facility or delivery area.

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 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.

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

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.

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.