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ZIP Code Database - Facts & Stats

ZIP Codes: Map 1963 Through Today

What is a ZIP Code?

A ZIP code is a numbering system with separate code numbers for all cities in the United States.

On April 30, 1963, Postmaster General John A. Gronouski announced that the ZIP Code would begin on July 1, 1963. The United States Post Office Department introduced the Zone Improvement Plan (ZIP) Code, a coding system that assigned codes on maps to all addresses in the country.

In 1943 the United States Post Office Department divided cities into zones to assist in speeding sorting and mail delivery. By July 1963, a five-digit zip code had been assigned to every address throughout the United States map.

What do the ZIP Code Digits Stand For?

There are five digits in the original ZIP Codes. The first digit indicates one of ten large geographic areas in the country, ranging from zero in the Northest to nine in the far West. The second and third digits indicate metropolitan areas and sectional centers accessible to common transportation.The fourth and fifth ZIP Code digits indicate local post offices or postal zones in larger cities.

How are ZIP Codes used by the Post Office?

This new Zoning Improvment Plan coding system made it necessary to establish large transportation centers throughout the country to relieve major metropolitan post offices of the burden of processing all the mail.

In 1965, a high-speed optical reader was introduced by the United States Post Office Department. This machine could sort mail by reading the ZIP Code automatically. They read the address and printed a bar code on the envelope that corresponded to them. At destination post offices, a bar code sorter then read the bar code and sorted the letters by ZIP Code and address into appropriate holding areas to await delivery.

Sectional centers are where most mail is processed. A letter mailed at your local post office may be delivered to a sectional center if it isn’t destined for delivery within the same ZIP Code as you mailed it. Before delivery to local post offices for mail carriers to distribute mail to its final destination, automated systems sort the mail and postmark it.

For more information about the automation of the United States Postal Service, please see our article on ZIP Codes and automation.

Zip Codes Plus: The ZIP + 4 Code

In 1983, the United States Postal Service expanded the ZIP Code system to include four more digits. This is called the ZIP + 4 Code. The additional digits identify even more precisely the mail’s destination. According the the United States Post Office, the new sixth and seventh digits indicate a “delivery sector, such as several blocks, a group of streets, a group of post office boxes, several office buildings, or a small geographic area. The last two numbers denote a delivery segment, which might be one floor of an office building, one side of a street between intersecting streets, specific departments in a firm, or a group of post office boxes.”

This new ZIP + 4 Code speeded up mail handling by reducing the number of times a letter had to be handled, and reducing the time mail carriers spent placing their mail in the order of delivery. A multiline Optical Character Reader reads the address, then sprays a barcode representing the ZIP + 4 plus two additional digits indicating the exact delivery street address. Using this barcode database, it can sort the mail in the correct sequence for each carrier’s delivery route.

The Importance of the Correct ZIP Code

The United States Postal Service emphasizes the importance of addressing your mail correctly in order to assure timely delivery. According to the Postal Service, “Using the correct ZIP Code helps to direct your mail more efficiently and accurately.” They suggest you use a site such as to obtain the correct ZIP Code, as well as the correct spelling of the City and State. If you need to get a ZIP Code and don’t have access to the Internet, you can call 1·800·ASK·USPS and get it by phone. Using zip code maps can also help and may save you a dime on the call.