We send our mail according to the proper ZIP code. What is ZIP an abbreviation for?

The five-digit code we use to send mail was introduced by the United States Postal Service on July 1, 1963. Before this, the post (Review) office used a system of postal zones, but these were only in place for 124 of the largest cities. As the country's population grew and more people moved to suburban areas, the volume of mail dramatically increased, doubling between 1943 and 1962. This surge in mail made the old system inefficient. A new, more organized plan was needed to help sort and deliver the mail more effectively across the entire nation.

This new system was officially called the Zone Improvement Plan. The acronym was intentionally chosen to suggest that mail would travel more efficiently and quickly, or "zip along," to its destination when the code was used. To encourage the public to adopt the new codes, the post office even created a cartoon character named Mr. ZIP. The five digits of the code each serve a purpose; the first digit represents a general region of the country, from "0" in the east to "9" in the west. The next two numbers identify a more specific regional sorting facility, and the final two digits pinpoint the local post office that will deliver the mail. In 1983, the system was expanded with the ZIP+4, adding four more digits to designate a location as specific as a city block or a group of apartments.