Input (grade 6): In 1983, the U.S. Postal Service began using an expanded ZIP code system called ZIP +4, often called "plus-four codes", "add-on codes", or "add ons."
Output (grade 5): In 1983, the U.S. Postal Service began using an expanded ZIP code system called "ZIP + 4", often called "plus-four codes" or "add-on codes."