A ZIP Code is a five-digit number that identifies a specific area where mail gets delivered in the United States. When you address a letter or package, you write the ZIP Code after the city and state, like “Springfield, IL 62701.” The numbers help postal workers sort millions of pieces of mail quickly and get them to the right neighborhood.

The U.S. Postal Service created ZIP Codes in 1963 to speed up mail delivery. Before ZIP Codes, postal workers had to memorize which neighborhoods belonged to which delivery routes. With ZIP Codes, machines could read the numbers and sort mail automatically.

Each ZIP Code covers a particular geographic area. Sometimes a single ZIP Code serves one small town. In big cities, different neighborhoods have different ZIP Codes. Very large buildings, like the Empire State Building, sometimes have their own unique ZIP Code because they receive so much mail.

The first digit of a ZIP Code identifies a broad region of the country. For example, ZIP Codes starting with 0 are in the Northeast, while those starting with 9 are on the West Coast. The next two digits narrow it down to a smaller area, and the final two digits pinpoint a specific post office or delivery zone. Some addresses also use “ZIP+4” codes, which add four more digits for even more precise sorting.