ZIP code stands for Zone Improvement Plan.

Before 1963, postal workers sorted mail by reading city names and street addresses by hand. This process was slow and became difficult as the amount of mail grew after World War II. Robert Moon, a postal inspector, suggested using numbers to represent mail centers in 1944.The five-digit ZIP code system organizes the country into regions. The first digit represents a large area, starting with 0 in the Northeast and ending with 9 in the West. The next two digits identify a regional mail hub. The last two digits point to a specific local post office.In 1983, the Post Office added four more digits called ZIP+4. These extra numbers identify specific city blocks or large buildings. This allows machines to sort mail even more accurately.Modern machines called Optical Character Readers scan these codes to sort mail. These machines can process over 30,000 pieces of mail every hour. In the 1950s, a person could only sort about 50 pieces of mail per minute by hand.