In 1983, the U.S. Postal Service introduced an expanded ZIP Code system that it called ZIP+4, often called "plus-four codes", "add-on codes", or "add-ons".
1983年，美國郵政開始使用一套改良了的ZIP編號，稱為"ZIP+4"，通常被簡稱為「加四編號」或「添加編號」。