<P> In 1971 Elmira (NY) Star - Gazette reporter Dick Baumbach found out the White House was not using a ZIP Code on its envelopes . Herb Klein, special assistant to President Nixon, responded by saying the next printing of envelopes would include the ZIP Code . That did occur . </P> <P> 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". A ZIP + 4 Code uses the basic five - digit code plus four additional digits to identify a geographic segment within the five - digit delivery area, such as a city block, a group of apartments, an individual high - volume receiver of mail, a post office box, or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery . However, initial attempts to promote universal use of the new format met with public resistance and today the plus - four code is not required . In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP + 4 Code from the address--along with the even more specific delivery point--and sprays an Intelligent Mail barcode (IM) on the face of the mail piece that corresponds to 11 digits--nine for the ZIP + 4 Code and two for the delivery point . </P> <P> For Post Office Boxes, the general (but not invariable) rule is that each box has its own ZIP + 4 code . The add - on code is often one of the following: the last four digits of the box number (e.g. PO Box 107050, Albany, NY 12201 - 7050), zero plus the last three digits of the box number (e.g., PO Box 17727, Eagle River, AK 99577 - 0727), or, if the box number consists of fewer than four digits, enough zeros are attached to the front of the box number to produce a four - digit number (e.g., PO Box 77, Juneau, AK 99750 - 0077). However, there is no uniform rule, so the ZIP + 4 Code must be looked up individually for each box . </P> <P> The ZIP Code is often translated into an Intelligent Mail barcode that is printed on the mailpiece to make it easier for automated machines to sort . A barcode can be printed by the sender (some word - processing programs such as WordPerfect and Microsoft Word include the feature--but is not recommended as the address to ZIP lookup tables can be significantly out of date). It is better to let the post office put one on when it processes the piece . In general, the post office uses OCR technology, though in some cases a human might have to read and enter the address . </P>

What are the last four digits in a zip code