Original: 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 . ''
Simplified: 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 . ''