The ZIP Code was instituted in 1963 and allows for electronic processing and delivery of mail. An envelope that does not include a ZIP Code in the delivery address must be manually sorted, which increases the cost of sorting the mail and causes mail to be delayed en route to the delivery address. ZIP Code directories are available for use or sale at your local post office, or you can look up ZIP Codes on-line.
In 1983, the Postal Service began to use an expanded ZIP Code called ZIP+4. It is composed of the original five-digit code plus a four-digit add-on. The four-digit add-on number identifies a geographic segment within the five-digit delivery area such as a city block, an office building, an individual high-volume receiver of mail, or any other unit that would aid efficient mail sorting and delivery.