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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.

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.

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.

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).

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). If you are validating US postal codes, allow for both conditions. The following rule validates a field that can be five digits, five digits + dash + 4 digits, or blank.

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.