There is also another type of postal code, the ZIP + 4 code. According to unitedstateszipcodes.org, the ZIP + 4 was established in 1983 and identifies an even more specific location within a ZIP code.

Question: My son is insisting that I can send him postcards to his name and 9-digit ZIP code, with no street address included. I don’t believe him. What’s the story and how do ZIP+4 ZIP… ...

Over the next two decades the ZIP system became strained, too, and in 1983, the Postal Service expanded it to create the ZIP+4 system, tacking on an extra four digits at the end of the old codes.